Cheers folks - great info
Guy - I'm not looking at disposing of the alloy mirrors. I want to keep them for the summer and run the plastic for the winter, or if I can't be bothered to change them back at least have them for when we move house to somewhere with a double garage with I don't have to worry about them getting stolen again!
I'm going to take a punt on the ones I've found being the right type - just need to agree a price that suits, in case they turn out not to be correct.