You can buy solenoids seperately I would imagine, as its probably just a generic boost control solenoid.

Even the autronic ones are only 100 quid, so cant imagine it being anymore than that as autronic arent known for under pricing stuff

Edit: here you go, will be one of these
http://www.blitz-uk.co.uk/ProductItem-318.aspx
http://www.blitz-uk.co.uk/ProductItem-319.aspx