Same happened to my Astra years ago, got a fresh cover from a scrapyard for a fiver and it clipped straight on but I had to spray it up as I got it off a white Astra and mine was purple (don't ask, it looked good in the showroom)
Scrapyards don't need to be local. Go to this site and the scrapyards come to you.
www.partsgateway.co.uk
I am in Belfast but have bought bits from scrapyards from Barnsley to Stanford-no-Hope in Essex via this site. Reasonable delivery charges for small items like this, not sure what they'd charge if you were looking for a gearbox or back axle though.