At the service two weeks ago the independant garage that I've started using highlighted an issue with a pulley bearing on the auxillary belt. This sounded much better than BMW's diagnosis - "that'll be the £600.00 aircon pump on its way out". Been meaning to book it in to get it looked at in more detail. Got around to it yesterday after the car had got pretty hot the previous evening whilst I sat stationary on the side of the road with the engine running for about 15 minutes (the engine's so quiet I hadn't realised I'd left it on!!).

Just had a call back from the garage and it is the pulley bearing which'll cost about 20.00 for the part. The heat problem was as I suspected; the front radiator fan wasn't running. 120.00 for a new viscous coupling which hopefully BMW have in stock. An hour's labour at a third of BMW's rate and they're going to adjust up the handbrake at the same time too.

All in all a pretty good result.