If you haven’t bought your 2016 Fuller’s Vintage Ale yet, either to drink now, or to lay down for later, or to preserve as an investment (what with examples from the 1990s selling for up to £500 a bottle, and even the 2013 costing £40 a pop), tough tubas – there’s none left. Waitrose is totally sold out, so is the brewery shop. Luckily I had a hunch my local specialist, Noble Green in Hampton Hill, might have some, and I manage to snaffle their last five examples.

Fuller’s is being tight-lipped about why the 2016 is now impossible to find: there are rumours that something went terribly wrong with the packaging, but no one seems willing to say. It’s a great pity, because the 20th iteration of Vintage Ale since it was first brewed in 1997, is a lovely, lovely beer, already, at approaching a year old, deep and remarkable. This was the one with Nelson Sauvin as both a boil hop and an FV addition, the first time, I believe, that Fuller’s has used New Zealand hops in VA, and it works brilliantly: there’s limes coming through, and passionfruit, and mandarins, and a little bit of that Nelson Sauvin elderflower, all beautifully integrated over creamy toffee and deep brown malt sweetness, with just enough bitter (40 IBUs) to hold everything together. You’ll drink one bottle, and enjoy teasing out all the flavours so much you’ll want another one to continue the analytical fun, and then at the end of that one you’ll stand up and wobble slightly and realise you’ve just drunk a litre of 8.5 per cent ale.

How the 2016 will develop as it gains more age remains to be seen, but Fuller’s had a gathering in the Hock Cellar at the brewery a couple of weeks back to celebrate the 20th anniversary of Vintage Ale with a tasting of ten different examples going back to 1999, and all are still very drinkable. John Keeling, Fuller’s brewing director, who helped the late Reg Drury brew the first Vintage Ale in 1997, conducted the tasting and revealed a few secrets about the beer. Vintage Ale was, he said, an idea first put forward by the marketing department at the brewery – “they do get a good idea every 40 years or so.” However, Fuller’s knew something like Vintage Ale was possible after bringing out 1845, a bottle-conditioned strong ale made originally to celebrate the 150th anniversary of the Fuller, Smith and Turner partnership in 1995, and discovering that it actually tasted better at 12 months old than when it was new – “totally the opposite to every other beer at that time”.

A beer has to be specifically designed to age, Keeling said: “Most beers will not age properly.” After 20 years, Fuller’s now has considerable experience in how beers age, with the interplay of negative reactions – notably oxidation – and a whole series of generally more positive chemical changes, such as Maillard reactions between sugars and proteins, which happen at different speeds, while at the same time alpha acids are breaking down, reducing the perceived bitterness (and boosting the perceived sweetness) and adding extra complexity of flavour, the colour of the beer is darkening and “madeira” and “sherry” flavours start appearing, and eventually “cherry” flavours, which you can cerrtainly spot in the older Vas.. The different speeds that the “good” and “bad” reactions take place at gives a “cycle” to beer ageing, which explains why that bottle of 2013 VA may taste disappointing now, but one of its brothers will be terrific if left for another nine months – and a third bottle of the same brew will disappoint another nine months after that, which a fourth, left for longer yet, will again cheer and enchant as it comes back “on” … you can regard this lottery-like aspect of beer ageing as annoying or part of the fun, but it does mean you shouldn’t dump the whole batch just because one aged bottle is disappointing. It may be just at a poor spot in its cycle.

One important aspect of beer ageing is that temperature is important – and room temperature is the worst temperature to store beer at, Fullers has discovered. It appears the oxidation cycle at around 20C is happening too fast for the “good” cycles to compensate. Either keep the beer cool, or, counter-intuitively, keep it warm: with the warmer beer, the “good” reactions are speeded up more than the “bad” ones, so the oxidation is outpaced. (Doubtless this was the clue to the success of ship-borne India ales in the 18th and 19th centuries, with the oxidation of beer in the casks lagging behind all the Maillard reactions and so on made extra-fast by the warm Equatorial seawaters of the mid-Atlantic and the Indian Ocean.)

VA is always parti-gyled with London Pride, which raised a question: each year the recipe is altered slightly, with different hops and combinations of hops. Have Pride drinkers never noticed over the past two decades that every spring their beer tastes rather different, from the Fuggles and Target of 1999 to the all-Goldings of 2002 (that year’s VA was always a personal favourite, and it’s still wondrously smooth aged 15), the Goldings, Liberty and Cascade of 2014 and last year’s Nelson Sauvin, Goldings, Northdown and Challenger? I’d love to know if anyone has ever commented … see if you can spot the “Vintage Ale” gyle this year.