They will confirm things like colour/paint code, any additions from new inc. anything like a top-tint windscreen (yes, that was an option!), engine and gearbox number (when new, in case they've been changed), actual date of manufacture ie. when your Porsche rolled off the production line, confirm UK market/RHD (or not) of the car.
If you want it all put onto official Porsche paper then there's a £64 fee to pay, but if you're happy to jot it all down over the phone, the information is free.
You will have to fax/scan/email a copy of your V5 registration document, to prove your connection with the car, before they will divulge any information.
Best method of contact is to email an enquiry via the Porsche GB Ltd website. They came back to me by telephone within 48 hours and after scanning and emailing the V5, they called me again almost immediately.
EDIT: and if you want to be really nerdy (as I've been), you can even complete a DVLA form asking for details of previous owners. They do however require a detailed explanation of why you want the information and what you intend on doing with it. In my case writing to the previous owners, particularly the first owner who kept the car for 14 years, asking if they could supply me background history, where they took the vehicle, any old photos (if they no longer want them) and very importantly, what their inspiration was behind choosing L998 colour to sample yellow.
