Brian, point out this one which isn't selling on ebay:
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=9860&item=4507467716&rd=1&tc=photo
The brutal fact is that 986 prices have taken a humping with the new model and the 550 isn't holding its value much better than a standard 'S' - so as painful as it sounds to 550 owners, your offer of £36K is probably about correct for this time of year - in spring it might be £1K more. Make the offer, let them refuse, in the cold light of January when no-one is buying and their 18 month warranty is now down to 16 months or less and time is marching on, then they will call you back and accept.
Although I do agree with others, phone around every single OPC in the country and you will find a boxster build in early spring - I'm sure of it, and I think the depreciation would be no more than a 550. I still see the 550 has a long way downward to go yet, and will settle at around £500-£1K more than a similarly specced 986S.