Try the whizzo new diet, they said. You’ll become fit as a grasshopper and acquire a complexion so luminous it glows in the dark like a scout’s wristwatch.

A week would be spent according to the dietary doctrine of Blairite heiress and food blogger Ella Woodward, willowy new pin-up of food fashionistas.

There’s no denying that she has struck a chord — her book, Deliciously Ella, released in January, was the fastest-selling debut cookbook since records began, beating Nigella Lawson’s How To Eat and Jamie Oliver’s The Naked Chef.

Her blog, where she posts weekly recipes, attracts 2.5 million hits a month and she has 250,000 followers on something called Instagram, plus 420,000 YouTube disciples who absorb the former model’s tips for clean living.

Ella is the new Nigella but without the sugar, curves or much else. As her book promises, I would be entering a world of ‘awesome ingredients and incredible food that you and your body will love’.

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Popular: Food blogger Ella Woodward's book is fastest selling debut cookbook, beating Nigella Lawson’s How To Eat and Jamie Oliver’s The Naked Chef

Awesome, incredible, love: these, one discovers, are very Ella words. They make up for some of the things that are not found in her larder.

For fey, slender Ella — whose dad Shaun Woodward is a Labour MP and mum a member of the Sainsbury dynasty — is not just a vegetarian. She is not just a vegan. She is a plant food fanatic.

Under her regime there is no red meat or chicken. Fish and seafood are out, as are milk, cream, cheese, yoghurt and other dairy products.

Eggs? Verboten. Ditto refined sugar/sweets/most chocolate. This being a gluten-free operation, you may not eat flour or other wheat derivatives or anything genetically modified.

She is against tofu, too. Oh, and alcohol is pretty much a no-no.

Once a month, cautious Ella might risk a few snorts of top-quality vodka but the rest of the time she is a water and fruit juice girl. Having given up wine and spirits for Lent, I did not mind about that.

So what can one swallow? Well, it helps if you like avocados. She uses a lot of them.

Dates also feature large in Ella recipes, though they must be medjool — fatter, juicier and more expensive than the shrivelled bullets you find in those oblong boxes of Tunisian dates at Christmas.

We visited a health food shop in our local town, Ross-on-Wye — I couldn’t help noticing how unhealthy the customers looked — and bought its entire stock of, er, ten medjools.

They cost £3.50 and were enough for just one recipe. Eating a la Ella is not for paupers and it may be easier if you live in a big town where posh ingredients are more easily found.

In the first two days we spent £75 on what looked disconcertingly like bird food, plus a carton of quite disgusting almond milk. My wife Lois and I stared at the provisions and gulped — in trepidation rather than relish. We would not be doing much relishing for the next week.

Lacking variety: Food blogger Ella Woodward's book features 'endless nuts'. Hazelnuts, almonds, cashews, brazils and pecans. But no peanuts

In front of us were bananas, berries, buckwheat (a cousin of knotweed), chickpeas, coconut milk, limes, oats, quinoa (like tumbleweed, even if it sounds like a Fijian rugby player), seeds and sundried tomatoes.

Then there were the nuts. Endless nuts. Hazelnuts, almonds, cashews, brazils and pecans. But no peanuts. ‘Peanuts are susceptible to mould and fungal invasions which can be carcinogenic,’ writes former model Ella, whose body is her temple.

Lord knows what her proud father makes of all this. When I first knew sometime That’s Life team member Shaun Woodward in the Nineties he was a Tory spin doctor who smoked fat cigars and took me to lunch at London’s gusset-stretching Caprice restaurant.

He became a Tory MP, but defected to Labour in 1999 and became Northern Ireland Secretary.

Having married money in the late Eighties, he is one of the few Labour MPs to have a butler.

His 23-year-old daughter turned to this extreme diet in desperation four years ago when she was struck down by an illness called POTS (Postural Tachycardia Syndrome — it attacks the nervous system).

All other treatments had failed but the diet put her right. Before that, Ella had been a sugar addict, devoted to Haribo and Ben & Jerry’s cookie dough ice cream.

Now she lives on plants and not much else — and has never felt more full of beans. Literally.

Diet day one arrived. I normally take Lois a dawn cup of coffee in bed and had already sploshed in the milk when I remembered that dairy products were banned.

We had to have ginger tea instead. What about breakfast? Porridge would need to be made without milk or sugar.

‘I feel like a bacon sandwich,’ wailed Lois. Just thinking about it made us miserable. We settled for a couple of tangerines and a ramekin of dry granola (made with oats and dried fruit — out of a packet, I confess; Ella has her own recipe but we did not have time to make it).

As I headed for work, my tummy was already starting to rumble. Lunch was a mug of soup from the office canteen but I realised it had been made with chicken stock. Oops.

The salad bar would have been an option but the Greek salad with feta was no good, cheese being non-Ella-approved.

Nor may one smother salads with salad dressing or mayonnaise, which contain egg. Ella recommends carrot, orange and cashew salad, or a warm wild rice salad with pine nuts — it is given a flicker of life with a few squirts of tamari, a type of soy sauce.

After ten hours, Lois, who has a heartier appetite than me, was starting to moan with hunger.

She threw herself into Ella’s recipes. A lot of these are cold: cucumber and avocado rolls, fruit smoothies and a quinoa tabbouleh which is like a spongy couscous, though less filling, and with a bitter, meadowy taste not dissimilar to cress.

Ella’s guacamole, made with avocado, tomato, jalapeno pepper, coriander and lime juice, was a success, though not much different from any other guacamole. Ignore her hesitation about garlic and throw some in — it’s dull without it.

The same was true with Ella’s hummus recipe — the usual blending of chickpeas and olive oil and lemon juice and tahini (a paste made from sesame seeds, and far from cheap).

I bought some gluten-free lentil crackers to go with it. We were so hungry, we ate the whole lot in one sitting and felt a bit burpy afterwards.

We soon found this was a problem with the Ella diet: you become so desperate for grub, you binge on the few foodstuffs you are permitted. I powered my way through most of a packet of Marks & Sparks seed and nut mix one morning and afterwards felt like a dissolute squirrel.

In London, I found a box of deri dates and wolfed them down in a day. Cue several visits to the lavatory.

You soon weary of nuts. Their hardness, dryness and woody flavour started to grate. The weather in Herefordshire turned cold and we came in one day and realised we were shivering. We needed hot food — but what?

One of the good things about Ella recipes, with a few exceptions such as the homemade granola, is that they do not take much time. ‘Just as well,’ said Mrs Letts.

‘Thanks to her diet I haven’t got the energy to make anything more complicated.’

Within 20 minutes she had made Ella’s stuffed chestnut mushrooms — stuffed, that is, with sun-dried tomatoes, pine nuts, lime juice, basil and tahini.

It was a mercy to have something hot in one’s gut but the stuffing was far from delicious. The sun-dried tomatoes and lime were too acidic. Lois was blunt: ‘It tastes like sick.’

One of our terriers, Flip, was wagging her tail, doing her begging-eyebrows routine. I offered her my mushroom leftovers. I’m afraid they got a complete thumbs down. (Or should that be paws down?)

Our daughter Honor, 12, was unimpressed, too. Lois made a tray of Ella’s raw brownies — they look like normal chocolate brownies, but are made of pecans, dates, cacao powder and maple syrup. ‘Ew,’ came the 12-year-old’s verdict.

Deliciously Ella, released in January, was the fastest-selling debut cookbook since records began, beating Nigella Lawson’s How To Eat

I sympathised. The brownies looked good, but tasted acrid, waxy. Lois was so hungry, she marched through the lot in a few hours — and promptly turned as green as one of Ella’s cucumber and avocado rolls.

Undeterred, she made a pot of Ella’s cacao and hazelnut spread, another alleged treat. This looks like Nutella. You whizz baked hazelnuts, maple syrup and cacao powder in a blender (our 20-year-old Magimix was in constant use during the week).

From a distance it could have been Nutella, but it was grainy and had an almost waterproof texture.

Lois said: ‘Never mind, I have made a delicious key lime pie.’ So she had: almonds, dates and coconut oil for the base, the green filling being made of avocados, limes, maple syrup and coconut milk.

High marks for presentation. The texture was good. But then the moment of taste: again it had that waxy character, and the flavour of avocado overpowered the lime. My beloved spouse sought to boost morale by insisting it was terrific, and she had a second slice. I couldn’t finish my first.

The longer the week lasted — and, boy, it felt like a fortnight — the more we gorged on simple things: raw fruit, vegetables roasted in olive oil and, I confess, Walkers ready-salted crisps (so far as I could see, these were entirely legal under Ella rules).

Salt became quite a kick but I felt I was lacking something. What was it?

Finally I realised: it was onions. Princess Ella’s innards, which must be as spick and span as a royal yacht’s funnel, can’t tolerate onions. She does let you have them, but the lack of them in her recipes leaves a big hole, taste-wise.

How much better those salads would taste with slices of raw onion.

I went off-piste and made a lentil and cabbage mush-up. Normally I would use stock cubes but I flavoured it with garlic, cumin, turmeric and black onion seeds. Gosh, it was good.

We also scoffed a lot of oatcakes, too. I could not find Ella passing judgment anywhere on oatcakes, so we daringly presumed they must be OK.

Herbal tea, oatcake, seed snacks and then a main meal of stir-fried vegetables, or broccoli with a tahini dressing, or polenta with mushroom and kale: when this becomes your routine, morale is hard to sustain.

We looked at the photographs of Ella, so skinny and smiley, everything about her perfect, and felt incredibly depressed. Will she remain on this diet for her four score years and ten?

Perhaps it is easier to be so flawless if your parents are multi-millionaires. But give me the curvaceous, naughty-treats, smoky-voiced Nigella approach to life any day.

The end of the week approached and Glyn, our local butcher, asked what we would like in our weekly delivery. ‘Steak, sausages, bacon, chicken!’ yelled Lois, with the alacrity of a desperate cavewoman.

Actually, I did not feel so starved of meat. It was the eggs and cheese I was missing most. And a glass of vino collapso — but that will have to wait until the end of Lent.

Did we feel healthy? Was our skin lustrous? Not really. We felt quite pious, though.

And the final question: had we lost weight? I was perhaps a pound lighter at the end of the week. But Lois had put on three pounds. Not pleased.