I have the exact same gearbox in the 630i Owen. Around Christmas time the same symptoms were observed. It was the pump in the end, exact same part number for 630i SMG pump as the 330i.

The labour is a hell of a lot easier than it is on the ///M cars too.

Part number: 23 42 7 571 297


http://bmwfans.info/parts-catalog/23427571297/

As I mentioned I would buy it on CarPart-Pros.com

Price: € 294.80, plus €17 delivery, plus €10 PayPal fee (€380 plus VAT in the dealers)

You need the fluid as well. I would leave it to Mick C if you can get him to do it.

BTW: I have Rheingold too.