To be honest it is a tw@t of a job to do, when i used to take front pipes off to the turbo we used to warm the nuts and threds slightly but this is hard to do on some cars due to lake of space, as alreay said they, been a garage you would expect them to have experience of this and pre warn you. Possibly best option is to say that to them and try and just go 50/50 and pay for 2 hrs labour.
Just think if you had attempted it yourself on your back on the drive and they had snapped!!! The problem is now theres to fix.
Hope all goes well bud.