I think I'm being rather dim, but I assume I go to my local VX garage and give the registration or vin of the vehicle and they can then give me a car pass with the correct code?
Yup, registration is enough. Will take a day or two to actually get the details, its totally random if you actually get the propper card or not.
VX dealers require not only the V5, but some other form of identification as well, ie: driving licence or passport, as proof of ownership.
You also have to fill in and sign a form stating you are the registered owner, and they photocopy all the documents before they aquire the pass for you, to cover themselves against fraudulent theft.
I know, as I had a similar problem. Tried two different VX dealers, both demanded the same paperwork before they could get the pass.
Only difference was the price, 1 wanted £45 + VAT... the other £25 + VAT. First one said it would take a week !! 2nd one 48 hrs. (they had a paperwork code sheet 24hrs later, and got a proper plastic pass 2 weeks later.)
I actually needed the key code rather than the radio code (which was incorrect, as I swapped the crappy car400 for a CCR600/CDC2 with the codes)
Hope this clarifies the situation. 
(didn't want you to turn up with just the car reg/vin and be dissapointed)
You must have had a
very picky dealer, all i did with our local was ring up, say i wanted one, give the reg, job done. Picked up a week later with no identification.....