Ali_Z4C, thanks! All symptoms seem to indicate a faulty regulator.
From what I've read it seems to be the clips that are failing and although BMW don't sell them seperately you can get aftermarket ones on ebay for a few quid...
http://cgi.ebay.co.uk/BMW-WINDOW-REGULATOR-SLIDER-CLIPS-E32-E34-E36-E92-3-5-7_W0QQitemZ190341702150QQcmdZViewItemQQptZUK_CarsParts_Vehicles_CarParts_SM?hash=item2c513fe206
Do you guys think I should just get the whole thing replaced or try just replacing the clips?