if I wanted something different I should just write the details on the repeat slip before dropping it in to the surgery;

Oh, that's good! My surgery is rubbish about that, I can't change anything on my repeat slip, not even the number of things that I want.

Boots told me that once I got a pre-payment certificate

I don't suppose my Cunning Trick would work as well for you. The trick being to get a 4 month prepayment, then in that 4 months get the doctor to give you medicine for about 8-10 months. Then don't get another certificate until the stock has run down.