Cheers guys
The back story is, the driver's bulb did fail completely a few weeks back (I think it was an OEM Philips). I could tell because the little globe in the middle where the 'filament' is was all grey/black and burnt looking.
I had a couple of original Osram Nightbreakers in my toolbox from one of my old R32s, so I bunged them both in (I always replace things in pairs, don't ask why, OCD I think!) and all was good for a week, and now the passenger one is flickering occasionally, sometimes cutting out completely. Turning them off / on seems to bring it back online.
I suppose it could be a ballast, but thought I'd try the bulbs first. Is the ballast the red plastic box that twists on the bulb? Are they expensive?