Having been in the same situation as many posters here, I was also looking at a very expensive bill for replacement alloys, made worse by the fact that I would be spending all this money and still have the underlying problem.
Fortunately my local tyre shop recommended Shadow Wheels and Tyres in Reading (
www.shadow-wheels.co.uk) as having a good selection of lower cost alloy wheels and a repair service for small cracks in alloy wheels. I decided to visit Shadow for an independent check before walking into the local main dealer to be told the inevitable.
Shadow checked all four wheels for cracks without charging. Very knowledgeable and friendly people. Fortunately it was only the one wheel with the flat tyre that had cracked. At a very reasonable £40 it was an easy decision to get my wheel was repaired and I can now enjoy my BMW again knowing that I can keep my nice alloys and if it happens again the cost will not bankrupt me!
PS – I have absolutely no connection with Shadow. Just posting this to possibly help others near Reading.