Dewi said:
The problem is it has no reg at the mo, the previous owner has taken his personal plate off it and put it on a bloody Suzuki!

Obvioulsy the data hasn't been changed though yet, it should go back on it's original number but we have to wait for it to be re-allocated to the car
This doesn't sound right. If he has submitted the correct paperwork then at the same time as he transferred his plate onto the Suzi the Zed will have reverted back to its age related plate complete with updated V5 etc. If he has simply took the 'physical' plates off the car and stuck them to the bumpers of his new one then he is jumping the gun and the Zed should still be displaying the plates until the DVLA send out the certs allowing both of you to have plates made up with the relevant marks. The Zed cannot be without a reg no at any point in time, it is either on the private plate or the age related. A phone call to Swansea should confirm which it is at the moment.
Regarding HPI checks, the one I got a couple of years ago showed all previous private plates that had been on the car so was clearly done on the vin not the plate which makes sense as plates are not static whereas vins are.