The Retention Scheme enables a number to be held apart from the vehicle it has been assigned to, for a 12 month period pending its re-assignment to another vehicle. Only the registered keeper of a vehicle is entitled to apply for a right of retention. Application form V778/1 Retention of Vehicle Registration Mark must be completed and the application fee is £25 plus an assignment fee of £80, bringing the total payable to £105 (at time of publication). Successful applicants are issued with a V778 Retention Document, valid for 12 months, which records details of the registration number, the grantee (the applicant), and nominee details where supplied in the application. If, after you have received your document, you wish to add or change nominee details, you will need to apply to DVLA, returning your document together with a fee of £25.
Once you have assigned the retained number to your, or your nominee's, vehicle you have fully exercised your rights under the terms of the retention facility. After assignment, entitlement to the number comes through being the keeper of the vehicle which has received it. Only the keeper is entitled to apply for further retention or transfer of the number.
If the vehicle which is receiving the retained number is already registered, its existing number will become void unless an application to retain or transfer that number is made at the same time.
Slappy I got that from reg transfers site mate.
You can keep your number on "retention" for up to 12 months by doing it that way.
Once you have bought the van you can transfer (through dvla) to your retained number the van's original number becomes void then, and by reading the above, it looks to me like you would only have to pay the once.
So as I read it you could transfer to another van antime within the 12 month period.