Just to follow up - my problem eventually turned out to be an intermittent fault in the heating element. But because it was intermittent it was very hard to catch - most of the time it was heating up, pumping out etc. normally (which was why I replaced the controller).
It wasn't economical to repair on a fairly old unit, so I am no longer a Neff owner.
Phil McKerracher
April 2009
I didn't need to change anything. In the end I cleaned out the area where it drains from as well as I could (without dismantling and moving the dishwasher), and since then it's run fine for the last 6 months.