Transfering a personal plate

[ianrb]

Recently traded in my old used car for a new (to me) used car, and wanted to move my personal plate from old to new, with the dealer submitting the paperwork. Last time I did this I received a retention certificate, which I sent to my local DVLA office, and they sent me a new tax disk & a bit of paper allowing me to get new plates made up, and at that point I could put the personal plate on my 'new' car.
This time I received a V5 for my 'new' car with the personal number, but as yet no tax disk. Can anyone tell me at what point I can put my personal plate on my 'new' car? The DVLA web site offers no help and I fear if I phone them I may loose the will to live.