Just a quick reminder to everybody that there is no law saying you have to pay for your prescription when it is issued to you at the pharmacy. So if you get a penalty charge notice, ask them on what basis they are charging you (or fining you, if you prefer, though it's not a fine). There is nothing to stop you saying that you simply haven't paid for it yet, and there is no legislation in place to charge you for late payments - it's not quite the same as overstaying your time in a car park.
Don't pay a penny if you are entitled, but if you have't obtained the correct documentation, such as the certificate, pay for the one prescription. Once it's paid they have no reason to charge you for anything.
I repeat, there is NOTHING to say you must have paid for your prescription at the time you received the medication so basically they're screwed if they try to take you to court later for the unpaid charge, because they have nothing to charge you for.
Incidentally, as for signing the form, I never have - most pharmacies just ask me if I pay for my prescriptions and then sign it themselves (I buy the annual prepayment card as I have to pay for mine). I don't know what they have signed, so where's the proof I didn't actually pay for it? Again, you don't have to prove you paid, there is no legislation that says you do!