SJBimmer, my car was not a cheap one. It has full BMW history and 1 careful owner.
Anyway, the damn light came on again and this time the mechanic says it can only be one of two things. First, the ECU, he wants to change this at a cost of about £1,000 for the ECU, plus 20 minutes labour. He also wants to take rocker cover off, again, and look inside. Then, if that doesn't work, he said it must be the valvetronic system jamming.
For the record, this is a top independent BMW specialist in the UK, not just some dodgy mechanic.