I have not been in any accidents or hit any potholes.
They printed out a copy of the wheel alignment stats and they are happy with all the numbers ( I don't have it with me right now)
So I left it with them for two days and they gave it back yesterday and no improvement with this problem. They suggested we put new michelin tires on them as the old ones (continentals) were a bit worn. So new tired from a different brand didn't make a difference either.
They don't know what else this could be and I don't either. Not sure whether I should take this up with someone higher up in bmw as I get the feeling it could be a warranty issue.