They've got the car back so yet another update.
Sytner have removed the dash again and driven around without it. They seem to think the noise is coming from what they call the 'dash carrier. Basically whatever the dash is attached to. So they've ordered another one but unfortunately it's got to come from Germany. They've had my car since Tuesday and don't expect the part to arrive until the middle of next week.
In the meantime I'm driving around in a Mini