Just to update, found a used mirror on eBay, £20, arrived two days later and fitted in 10 minutes!
Very happy, and well worth waiting for something to come up on eBay.
One thing to mention - it was advertised as a GS450 mirror and I'd been searching originally for GS300. Not all sellers will include all the models that their part fits, so it's worth bearing in mind if you find yourself in a similar situation.
Thanks all for your responses,
Andrew