There’s a wedding going on, someway near Slough, soon. On the same day as the FA Cup final, so I can’t imagine anyone’s going to be that fussed about it. Trevor McDonald and Julie Etchingham are excited, though. Sir Trevor remembers Charles and Diana’s wedding, when 600,000 people lined the streets. That was the problem with the marriage, too, wasn’t it, too many people in it?

This isn’t about them though, it’s about their second son, and his American bride, a modern wedding for different times. Julie and Sir Trevor are going behind the scenes to try to find out some secrets ...

Venue

Harry and Meghan will marry in church, even though the bride has been married before. That’s allowed now, without the need to behead or banish anyone. Charlotte, Chapter Clerk of St George’s Chapel, explains that the actual knot-tying will happen down that end, the usual end, with the altar. And the Queen will be sitting somewhere, Trevor doesn’t know exactly where, though.

Guests

Again, not really known. But Alexandra Shulman, who used to edit Vogue, was a guest at Harry’s older brother’s wedding, and here’s the tin that held her piece of cake. She’s eaten the cake. There will be some schoolchildren there, and Trevor and Julie have been invited to a local – Catholic! – school’s slightly creepy role play of how they think the ceremony might go.

Dress

Almost certainly white, otherwise top secret, of course. So here are some other royal brides in their white wedding dresses – Anne, Fergie, more marriages that didn’t last. And inevitably a chat with Elizabeth – not the Queen but Elizabeth, who with her (now ex) husband, made the most famous dress of all, for the woman whose marriage most famously didn’t work, Diana. Here is the actual pattern for that dress, which was laid on the fabric and cut out, much the same way as any dress is made. They had to be very careful and keep every scrap she says, because journalists were going through the bins to see what colour it was going to be. What colour was it? White.

Flowers

Myrtle probably, a tradition in royal bouquets. Meghan Myrtle. Also, peonies, careful how you say it. And roses. Plus, lily of the valley for its sweet fragrance. Lily of the Allen probably won’t be there, after her outrage (mock, she says) at not being invited to William and Kate’s.

Cake

McVitie’s, the biscuit people, usually get the job. They did a chocolate biscuit cake for Kate and William. They can’t tell Julie much though, the recipe is still a state secret, but Barry the chocolatier shows her how to make a white chocolate dahlia. The chocolate biscuit cake can’t have been that good as Meghan and Harry’s cake will be a lemon and elderflower one made in Hackney by a Californian called Claire. Hang on, is that something – pretty much the only thing in this sugary nonsense of a documentary – that is actually known about this wedding, rather than speculation or something about a previous wedding?

Transport

Horses ’n’ carriages, pomp ’n’ ceremony. Retired coachman Alfie rode one of the horses that pulled Andrew and Fergie on their big day. “Gosh, it’s going fast,” he says, in his deadpan, monotone way as he watches the old footage. Alfie can have a surname because he looks after horses and is called Alfie Oates. He is almost the best thing in this.

Bridesmaids

Guess what? It’s a secret. But in the Windsor school’s roleplay version, this little girl is playing one. What was it like, Julie asks her afterwards, how did she feel? “A bit gross,” says the girl. Ha! I love her, she is the best thing in this.

Annabel was one of Princess Margaret’s bridesmaids. She remembers the balcony afterwards, looking down on the crowds. “It was like lots of octopuses if you like, waving arms, it was just incredible.” That was another one that ended in divorce, wasn’t it? But hey, don’t let a bad marriage spoil a good wedding.

I hope Harry and Meghan are very happy, have a brilliant marriage and a fabulous day. She seems lovely and I am very much looking forward to the FA Cup final. Unless there’s a late invitation in the post, in which case I could probably make room for a bit of lemon and elderflower cake.