Thanks - in the end I chickened out and took it to the local dealer, and watched them struggle with trim, fiddly clips and taking the bulb housing off from the outside too. Guess it's easier when it's not your car - you're probably less wary of snapping a piece of plastic trim. What's wrong with a couple of screws holding the part in place? Anway, fair play to the garage; their service manager came out, swapped the bulb for me and only charged me the £1.60 for the bulb.
The warning on the dash display came on today and sure enough the high level brake bulb isn't illuminated. Page 70 of the driver's manual attempts to give instructions on removing trim to get to the bulb, but I just can't work it out from the instructions or pictures. Any help please?