You dont have to have the reg, it does make it quicker though. Send it with a covering letter giving all the old addresses, stick a cheque in for 25 quid and wait. mines been away for a month, maybe 6 weeks, they cashed the cheque last week so it should be on its way back.To use The V62 form, you have to know the reg number.
I’ve a friend here atm and we were discussing this. He reckons that a business HPI check might well be able to link a VIN to a reg number..