So this week I go to take it for a spin around the block and to get some petrol because I need to take it for an MOT. Once warm it ran like a total pile of crap. It wouldn't start at the petrol station and when I got it going it ran badly. Only 1.5 miles form my house so I made it home fine.
So this evening on checking everything over I thought it would be a good idea to check the distributor cap and rotor and to be honest I was not expecting this.

I am amazed how well it ran for the year given this was lurking in the engine bay. The end of the rotor is totally eroded. I have never seen a cap and rotor as bad as this. The photo really doesn't do it justice.
Anyway Dave has kindly loaned me a rotor arm so does anyone know what is currently the cheapest option for the cap?
I think there have been some posts recently about cross referencing BMW items (or P/Ns)