thanks paul, ideally i dont want to to so much effort to as its on 119,000 and not worth spending on...
but i apreciate your reply
Fair enough, but I don't think the part you want is available seperately. The ebay route for a secondhand one is an option, you sometimes see mirrors without the rear cap cheaper, and a scrappers might let you have an incomplete one cheaper.
Could the old one be repaired with epoxy resin, or is it too far gone? Wilkos do some small tins of matt black metal paint which will overpaint repairs.