Finally got the 4-wheel alignment I paid for! In the end I had to take the rods off and get them in a vice to get the lock nuts moving.
Obviously, having had the rods off, the toe-in was some way out and I've got to say, it was quite an eye-opener how bad the handling was! The car felt extremely unstable, so much so it was plain scary to drive at speed around the mildest of corners.
Once back at the garage for another go on the alignment machine, I find out that the guy responsible for the first couple of failed attempts was a stand-in. The guy who normally does the alignment had been off work and it turns out, he does know what he's doing! So, all sorted now apart from right rear camber being a touch out of tolerance.
Just want to say thanks for all the advice I got from you guys - much appreciated!