Graham
I think I understand what your talking about, is it the bolt that holds the elbow to the head that now turns with little or no effect?
If so then you may have shagged the thread in the alloy head, I am not sure wether you can drill and re-tap the hole to take a bigger bolt, the proper way to do this is to fit a helicoil (found this on the web, may help explaine the method
http://www.mantaclub.org/forum/topic.asp?TOPIC_ID=1858 )
Is it the elbow closest to the front or the back of the car, both would be easier with the head off, but if it is the one closest to the front I don't think you will have much room to work so it may have to come off.
I think the standard engine comes with bolts to hold the elbow on, I have found it better to fit studs and fix on with nuts, at one time I had a collection of 500 and 126 heads (and one or two blocks) that all either had the threads stripped or had snapped bolts stuck in them, you have to be very gental with steel bolts in alloy heads and blocks