Thank you for quick diagnosis - of course that makes sense - the stem at the end of the barrel must have sheared off. I found a Russian video on YOUTUBE which has pretty good demonstration of how to fix (even though I did not understand the spoken commentary). Managed to remove the barrel - sure enough, the end of the drive pin had broken off. It must still be stuck inside the receiver part on the left-hand side of the steering column. That is now my problem - removing the receiver part. The electrical contacts simply pulled off, but I need to reach the small grub screw which holds the receiver in place, and there is no room. The Russian vid has less plastic clutter (all modern gizmos are clutter as far as I am concerned) attached to the steering column. I need a tiny screwdriver with a right-angled head and a ratchet drive! Otherwise might have to remove steering wheel - the hole gets deeper, but at least there is some light now ... Thanks again for your help :-)

Another possibility is to install the new parts , and hold the old key up to the immobilser pick up ring ,when you want to start the engine. Once started ,the old key can be removed away from the vicinity of the pick up point. The coded immobiliser chip is only needed for starting, not needed for running.

Good progress and ultimately success, though all a bit tricky. You were right about the immobiliser code of the new part not working. I had hoped it would work as the new part includes both the key with transponder and the pick-up ring on the barrel. But even if they do match each other, they are obviously sending the wrong code to the ECM. So using plan B, have inserted old key into old part and left it wrapped in neoprene cushion behind the dashboard. No chance of losing the old key that way (have no spare). Lack of Russian meant I missed the bit in the video about filing to a diagonal one of the projecting parts of the new barrel so that it could slip over the sprung steering lock mechanism. Got that figured, but still needed to file of yet another small part (again due to steering lock mechanism) as otherwise could not insert barrel with key at position II as required (and as instructed in the workshop manual - found online – not very helpful on this occasion). Remembered at last minute to set receiving part at key position II before reinstalling, so that it could accept drive pin at position II. All installed okay at last – and went for a drive! So now have new key for ignition lock and a copy of old key for door locks. Bonus: I can have copies of these made and they will all work! I just hope there is no disadvantage to leaving the old transponder key in the old barrel with old pick-up ring behind the dash. Maybe a flat battery? We will see... If can be disheartening finding this kind of problem, but your help made it all seem less daunting, so many thanks again for your good advice and tips :-)