Dek, I have no idea of the radiator being from a BMW they may be right but you could end up with the same problem again of course going that route. I nave a Mulfab aluminium one fitted on mine which has been in service for 5 years and SimonH on here produces his own also and both products come recommended. Button on here has recently purchased an alu Chinese version but I think he has only just installed it so a bit too early so say how that turned out.
Williams supply one which is probably made by either of the mentioned recommended companies.
Alu rad