L322 2006 intermittent ignition fault

Has anyone come across an issue whereby when you try n start the engine, all you get is a faint, rapid beeping from under the trim around the igntion key barrel? (At least that's where it sounds like its coming from)

We've tried it with both keys, relocking & unlocking the alarm/doors etc... And yes, the car is in Park too!

The strange thing is, it just happens at random. Sometimes will cure itself within a few seconds, and sometimes half an hour or so later.

Thanks in advance for any nuggets of wisdom you can help with.

Oh and another strange thing about it, it only seems to happen when the seat is in memory 1 position (way forward so the mrs can reach the pedals ) - although I'm sure that is just a coincidence, I thought I'd mention it in case it has any bearing.