In reply to jules699: If the cables where they run through routers are corroded this could cause your problem. Detach the cable from the rear mech and expose as much of the cables as you can. WD40 might help then grease cables before re-assembling.
In reply to jules699: Apparently the whole mech/derailleur was bent so had to be replaced. I'd not had a crash so bit of a mystery and now after taking it to my local bike repair shop I'm £50 lighter