I've had a response from the customer this evening and wanted to update everyone.
After the email I sent to him on Sunday, he decided to go and buy the required parts from Bmw on Monday morning.
He's spent the last few days fitting the parts himself and is happy that this has in fact cured the fault.
He's now happy to accept my offer of paying for the parts in full and final settlement of the issue and will put that in writing to me.
I'd like to say a massive thanks to everyone that offered me help and advice with this issue.
It's very much appreciated.