Well pleased to report this has now been sorted.
Dealer Service Manager phoned to say the cost would be 600 quid to put right. I told him, sorry but I aint paying that. So he then went on to explain 'these things happen' etc etc. I said sorry, Im not having that. Told him they had a duty of care where Im buying a service from them, the problem happened on their watch and, I wanted my vehicle to be put back to the condition it was in when it came in for the service and work.
He tried to say it could have happened at another garage to which I responded sorry, it wasnt in another garage, it had been serviced from new by them including the last cam belt and tensioner change. Anyway, if it had of been another garage, I'd still expect them to put my car back into the original condition. We then 'discussed' the finer points of consumer law, my intention to contact Vauxhall HO and a solicitor.
He said he'd have to discuss with the MD and I told him, Im happy to discuss with the MD too!
Got a call about an hour later saying due to my customer 'loyalty,' they'd do the work FOC but in no way accepted responsibility for the stripped thread. Fine.
Got a call on Friday to say the car was ready, cost over a grand!!

WTF!! I went ballistic and said all this had been agreed I wouldnt be paying and the work was FOC. Turns out someone hadnt been kept in the loop.
They delivered the car to my home and even valetted it! That was nice of them.
Thanks to everyone for advice!