If its in the middle it might be repairable. I had one that was repaired recently (along with the 3 that were replaced), the nail was in the centre so the garage said it was ok. You'll find a thread by me on this in the tyres/wheels section.
As it happened, i was in the Blackburn area a couple of months ago when i found a nail!! I was in Clitheroe, and managed to get it repaired by The Tyremen, Parsons Lane (if i recall correctly). They were great, repaired it for a tenner, and treated my car and wheels with respect. So i highly recommend them!
Anyway, back on topic, any more advice out there on wing mirrors? Can i fit it myself?
Through a friend of a friend, i've managed to get the part at cost price, still £67.00 mind.