Thanks for the replies, chaps, I'm giving them careful consideration. (160 views before anyone had any suggestions..! At least I'm not asking faqs, eh?
)
What I think I'm going to do is remove the cylinder head (I only snugged down the c/h bolts), put a dab of tippex (correction fluid) on the base of each stud and onto the c/h, and see whether the studs turn when I try turning a nut onto them.
I was at our local Volvo dealer yesterday (unrelated cause), and asked if they perhaps had some relic mechanic somewhere with experience of cars before they were all modular and digital. They're splendid people there, but the best they had that day was some guy in his thirties (I guess) who didn't have grubby fingers, and all he could say was "Well, they shouldn't be a problem..."
Thanks again for the contributions. Much appreciated.
I'll report back later.