Just to update this thread...
I contacted BMW UK and asked when the 320d would be available and they kindly responded as follows
Dear Mr ...............
Thank you for your patience whilst I find the answer to your query. *
I am pleased to advise that we will be releasing a diesel version of the BMW 320 xDrive, however, we do not have a confirmed date of the release at present. *We are expecting this vehicle to become available in early 2012, as the specification details are being finalised in January 2013.
I am sorry for the disappointment that this may cause you, however, I do hope that you may be able to hold on before replacing your existing vehicle.* If I can offer any further assistance, please do not hesitate to contact me again.
Obviously, early 2012 is a typo for early 2013.
August 2012 F10 520d SE sport automatic, Jet Black, Oyster\Black Leather, light ash wood trim, Professional media/ navigation, mobile apps, Internet, DAB radio.