I’ve done a quick search and found a company Called GM Suspension Technology, does any body have any experience of using them or any other suggestions.
Mike
Talk with John at JSR Tel: 01384 240890
Thanks Andy, i was actually speaking with John on Saturday and never thought to ask, so sorted now.
Mike
Just removed the rears from the car and not only is the adjuster seized but the spring platforms both snapped in half when i removed the springs 
under closer inspection both rings had lots of cracks so its a good job i was checking them.
Mike