What a saga this turned into, I'l summarise, but really happy its sorted and the way teh car is now!
car over heated, checked head gasket gone, quoted £900 for job including timing belt, water pump and having the cylinder head tested.
Cylinder head internally cracked and un repairable, sourced a recon unit for £650, added to original bill @ cost.
At this point 2 weeks had passed due to various issues.
New head fitted and car was about to be returned, but garage took car for road test and a valve fell out of the recon head while car was being driven - nightmare!
recon head was ruined along with a piston and a rod. Garage starts dispute with parts supplier and after 2 weeks of phone calls, photos of damage etc. Parts supplier agreed their cylinder head was at fault and that they would supply a second head, piston, rod and pay for labour to strip the bottom of the engine to change piston.
New head couldn't be sent until damaged unit was returned. Second recon head arrived sent to be tested and fitted along with new piston and rod.
6 weeks after the initial overheating, car returned but garage said the turbo wasn't kicking in properly, suspected vacuum leak. But asked me to drive the car for the weekend as they were happy that the lack of power would be good to help bed in the head, piston etc more safely. Car was totally under powered and was dangerous to drive so I gave them it back the next day. They couldn't find a vacuum leak but suspected that carbon in the turbo had hardened due to the car sitting for 6 weeks and could only see the turbo moving a few mm instead of the 10 or so it is supposed to. They sent the turbo off to be cleaned and serviced, refitted and the power had improved but not resolved. They then moved to the vacuum tank and i think they said there is a one way valve that was blocked (but not 100%) sure, they also replaced all vacuum lines.
So after 7 weeks car finally fully returned and the power is unbelievably better that when the whole thing happened. They suspect the turbo has always had an issue and was only half pulling in and therefor not getting its full spool. as they had it cleaned up as part of their investigation they inadvertently fixed an unknown issue.
Turbo kick in is far more violent than before and on a short test along the motor way @ 75MPH on cruise, the car was returning 45 MPG were previously it would only do around 35mpg. I have 700 mile round trip next week so will be able to see if the mpg improvement is as good as it 1st seems.
Sorry for the novel above, but not having your car for 7 week is brutal. Just relived the car wasnt terminal, in fact if I had to pay for all of that it would have scrapped the car as it would have cost too much. garage even gave me a discount on the original price and it ended up being costing me £1300 for the above.