If its definately a BMW compressor have a look at this BMW website.
http://bmwfans.info/parts/catalog/E4..._conditioning/
and check out the exploded diagrams.
Unlike MG-Rover they list all O-rings & gaskets.
BMW seem to have set price for all O-rings on all parts, = €1.26 Euros each.
I have bought O-rings direct from BMW parts department in the past after being ripped off by a MG dealer who insisted that you can't repair ZTT/R7/BMW fuel pump/sender units.
They'd never heard of the common BMW fuel pump/sender fault of 'Dissolving O-rings!
He was then proved wrong and now hangs his head in shame!
TK