I found out what the problem was. I'd removed the shock-absorber and disconnected the tie bar to let a friend weld quite a large hole in the floor above (one of the fail items on the CT). He had touched the thread on the tie bar - quite gently I imagine - with his angle-grinder.

So it was simply damage to the thread that stopped me doing the adjustment.

I've cleaned the thread up reasonably well and now I can do the adjustment. It takes a little more force than is strictly desirable, but it is OK. The damage isn't enough to weaken the rod.

My apologies for not discovering something more technical. I'm now slowly working round the car to get the ride height right on all four corners. Doing the front changes the back (and vice-versa) so it's a slow process.

Thanks very much to you all for the suggestions!

Its often something simple.
It has to be for me, or I would never fix anything.