It certainly ended somewhere between 2001 and 2007. I remember where I was living when they stopped it.
When mail came twice a day, I used to get my post in the afternoon, consistently, and none in the morning, ever.
When they changed it, my post started turning up earlier, in the morning.
So it was weirdly an improvement. It seems strange now that there was ever two deliveries a day, given I get one letter a week at most now. (parcels are another matter, but again you don't need Royal Mail delivering parcels twice a day even if you get a lot, so the fact they did this with letters is weird)
Lots of people mentioning Neopets. I did have an account there but couldn't log back on when I tried a few years back, so I guess my account was deleted (or I've got my login details wrong)
Not easy to tell as I have a lot of old photo scans where I'm not sure of the original date. So let's try this terrible photo, which I think was the first on a roll taken sometime in the 1980s. I don't remember that bunny so I must have been very young.
Ok, looks like Fedia was having a moment - it's working now. Nothing to see here.