Yes it will cost you, but only 25 quid. The person you are buying it from will be listed on the certificate as the 'Grantee', in order for you to use this on a car of your choice in the future your name will need to appear on the certificate as the 'Nominee'.

Its simple to do, there's a space on the form to make the amendment, send it off with 25 quid and it'll be sent back to the Grantee with the new details. He then gives the certificate to you and job done.

Be aware tho that if you keep it on that certificate beyond the renewal date shown on it, the reminder for extending the Retention will still go to the Grantee until you assign the plate to your car.

Once its assigned to your car he is totally out of the picture from therein.