So, the steering saga arrives at a conclusion.
Thought I'd post what I've found today and where I've got to in case it helps anyone else.
The good news is I've adjusted my original rack, I spoke to Roger at Kiley Clinton in Birmingham, see:
http://www.steering-racks.co.uk/ who, after seeing pics of the rack and the angled teeth advises its a Quinton Hazell or Dana rack, he says it's unusual because of the helical pinion and encouraged me to carry on working on the bronze plug.
It was pinned horizontally, the marks on the top were just peens.
But some judicious drilling later, it was moving and I finally got it undone.
Underneath is a nylon bush which sits on the back of the rack, pinching up the bronze plug has removed the little bit of play, so the rack is back in my chassis and I can plan to get the engine back in.
What else did I learn: the herald high ratio racks (2.5 turns) from Rimmers still give a 25ft turning circle, so if anyone wanted to use one on an Elan, restrictors will likely turn it into a 1.5 turn rack, very quick but could you park with it?
The baby elan rack I've got from John Clegg needs reconditioning, I had ordered extenders from Kelsport but when measured alongside the QH rack, they're too long with track rod ends and I'd have to start cutting down the tie rods, my QH rack has one extender on the near side but it's half the length of the Kelsport ones, I wonder if that's what I need for the Elan rack?
My plan is to go with the QH rack to get the car back on the road, and in the meantime, I'll recondition the Elan rack if I can get the right extenders and restrictors.
However, I can't recommend Roger at Kiley Clinton highly enough, he knows his stuff and could have reconditioned either rack for me, but he encouraged me to try once more on the bronze plug, stating I could save some money if I could adjust it - you don't often get that from a supplier.
I've had an offer of another rack from another forum member, will look at that option if it helps me to get one good rack together for the day I need to swap out the QH rack.
Thanks for all your comments and input.
Cheers