The noise comes from the idler pulley on the compressor - not sure how normal this is but there's an idler pulley that turns when the engine is running, then a magnetic clutch that engages the compressor proper. It was a constant annoying whine that rose and fell with engine revs.
Mine wasn't keen on my first bit of wading, which I think contaminated the bearing. It's a weird one with (I think) two ballraces in one enclosure, so you can't just buy a spare off the shelf. A new bearing was £40 ish, so if you can change them without damaging the new one then you'll save a fair bit. I'm just aware that if I buy another and mess up again I'll be pretty close to the cost of having the main dealer sort it out!
Does anyone know if the 300TDI aircon compressor is shared with any other vehicles/engines? Just thinking it'd enable me to widen my searches for a good salvage one.