Get to a baseline where everything tallies up, then find a new OPC...
Have you escalated your issues to the DP yet?
I don't remember anything about drive belts needing replacing? Is that specific to the 3.4 or have I missed / forgotten something?
Nearest OPC apart from Guildford is Reading which is a significant distance, Guildford is within a 5 minute walk from where I work so it's really a pain to change.
Not raised it to the DP yet, I'm with the Service Manager at the moment but maybe I should go to the DP if I don't get things resolved.
Service book says the following for 911, 911 Turbo, Cayman & Boxster;
Additional Maintenance every 90,00km (60,00mls), but no later than every 6 years;
Replace drive belt
Replace fuel filter 911 Turbo
Controlled all-wheel drive 911 Turbo: Change oil