Thanks @PleaseReferToMeAsBritneySpears, they are excellent ideas.
I think I will ring and say I got a letter by mistake and because it was obviously a mistake, I don't know whether the unblinded information is mine or not so I am going to ignore it and stay in the trial and do the crossover trial. Hopefully they will go with this, given they want people to stay in the trial.
I am tempted to say nothing at all, but then if they have a record that I have been unblinded, I won't be given dates for the crossover trial in April.
So I think I'll ring on Monday and ask to speak to the site manager to sort it out. I'll say before we go any further, I don't want to know if the letter was accurate or not because I don't want to be unblinded. The only problem with this is that it's a double blind study isn't it, so they shouldn't know who has had it either. Oh dear, it's all a bit confusing.
If they insist that I can't do the crossover trial I will be really annoyed!