My problem is now resolved and everything is working fine. The Technician arrived as scheduled. His name was Eamon and he was prompt, efficient and courteous. The fix was very simple. Something on my account had to be reset centrally to complete my switch. Eamon phoned "Field Support" and arrange for this to be done, and it was within a few hours after Eamon left and I got a follow up call to check that this was all satisfactory.
So, all is well that ends well and thanks again for your help. I am of course left wondering as to why this vital last step was not done without all this intervention. This was a change initiated by Virgin (not by me) and your inability to finish the job led to much wasted time, expense and frustration.
Best wishes, Rob.