Nah you don't, blackcurrant sandwiches are well known for being extremly poisonous!!
Seriously though, I had exactly the same problem with a ham sandwich craving, went on for weeks on end. BUT keep telling yourself that it cannot possibly live up to the expectation that you have of it and as soon as it is gone how will you feel. That one moment really isn't worth the grief that it will cause and as Clare said there is plenty of time for blackcurrant sandwiches when you have achieved what you set out to.
Hang in there, know its tough.
Gx