Hi,
I can't recommend any dealers in that area, but I would suggest looking at external checks/ servicing from an independent specialist and also again independent warranty.
I spent a long time deciding on this and found two local specialist (JZM and Daniel James - both in Essex) offering mutli point pre purchase checks (in fact both places employed ex Porsche technicians), servicing costs are much lower but they have all Porsche diagnostics and use all OEM parts. They gave a full check list (same used by Porsche themselves) and even fixed a faulty cup holder (that's all they actually found wrong with the entire car!)
I used the online Warrentywise firm - much cheaper then renewing the Porsche warranty that had just expired on the 996 C4S I purchased - includes wear and tear failure and additional options such as tyres, Media etc can be added. you can take it to any garage and you can tweak the excess etc. Just a thought. Also check the max age of car that Porsche will warranty - most companies including Porsche only offer warranties up to 8 years old I think. Also Porsche insist on the 111 point check prior to the warranty and all faults must be rectified or no warranty. Another frustrating point is when I bought mine I was told the PCM2 unit had been repaired a few months before on the then running Porsche warranty. however as the warranty from Porsche had now expired the SatNav was no longer under warranty even though they had put a new unit in?? It works 100% but if a problem did happen it's less than 12 months old but not covered? All very odd.
I think a car that has gone through any manufactures warranty for body etc (e.g. 5 or 6 years old) no longer requires dealership servicing. Many local Porsche specialists are looked upon as sometimes even better, get to know you and your car over the years etc - some may disagree. And of course for piece of mind and if you have then cash then stick with the dealers. I personally have spent the cash I have saved on stuff such as on track days, complete wheel refurbs, pro car detailing etc