Hi Tom,
If you call your GP or their out of hours service and explain your problem they are usually in a position to speak to a nominated pharmacist (best if its your usual chemist) and to get an emergency prescription sorted.
Failing that go to your normal pharmacist, see the pharmacist themself, explain your position and I am sure with knowing you for a good period of time will issue you with the tablets to see you through until the prescription comes through.
If it takes so long to get a repeat prescription I'd be having words with your GP/practice manager as that is completely unacceptable.
Equally, if your pharmacist can't help you, if you take the packets from your 850mg and 500mg tablets in, they may be better advised to the options available to you.
Hope you get sorted, but don't wait until next week.