That's not good news at all about the injectors. Do you use any sort of injector cleaner at all? Might be worth getting some and giving it a double-dose then a fast run between Fiveways and Thetford Sainsburys roundabouts and back again, just to give the injectors a heavy duty clean, or at least start the heavy duty clean with a bit of force.
Alternatively, what about this place, not far from you :
https://www.yell.com/biz/universal-d...andon-1391696/
Don't know if they'd be any better on price but at least more local.