Well following Haynes I've just had my tailgate trim apart and I manged to break only 1 clip in the process!
The bulbs for reverse and rear fog lights look as if you can only get them through Volvo, never seen the like before. I did manage to knacker one reverse bulb whilst seeing what type it is so will have a part number and price from tomorrow. I will also put a guide to changing them together some time tomorrow as well.
As for having both rear fog lights lit up, it's possible but looks costly. The hole is sealed but could easily be modified with a dremel etc. There is power for the fog light bulb going to the connecter in the tailgate (plugged the other side in to check), yet the light unit has no wire to plug a bulb into. Would require butchering another light unit or some home made wiring to make it work. Asked my local dealer for a price for a LHD tail light unit and the price was £170. Could get that down to £140-150 from Rufe or Gavin though. Still it's a tad expensive.