Rob,
I've used an outfit on the Old Dalby Trading Estate a number of times and gthey have been very good - cheap and finfished pretty much on the date they promised. They have blasted (and painted) rocker covers, bashplate, alternator cover etc etc and all have been fine. Blasting and painting a pair of rocker covers was about £15 IIRC.
These are the peeps:
Belmont Metal Finishings
Unit 4b, Woodhill Industries, Nottingham Lane, Old Dalby, Melton Mowbray, Leicestershire LE14 3LX
Tel: 01664 822482
They used to be in the unit next to Colin Jalland, but I think he's moved back up the road a bit.
HTH
LC