From the dvla website
Release of another vehicle s keeper details
You can obtain the name and address of the registered keeper of a vehicle if you can show reasonable cause for needing the information.
Members of the public will need to complete form V888. Companies can either complete form VQ3 or apply in writing giving full details of the reason for the enquiry and the vehicle registration mark. Both forms are available from DVLA.
The fee for information at a specific date of event is 2.50 per vehicle. If further information is required, for instance, a copy of a document or additional keeper information, a fee of 5 is required per vehicle.
Send your application to: Vehicle Record Enquiries, Vehicle Customer Services, DVLA, Swansea SA99 1AJ.
It is an offence to unlawfully obtain personal data which is contrary to Section 55 of the Data Protection Act 1998. Unlimited fines in the Crown Court (or to a maximum of 5,000 in the Magistrates Court) exist as penalties in respect of these offences.
I’ve used it a few years ago to find someones address for a similar problem to the OP.
Failing that he’s advertising for a new full sus mtb on mtbe.co.uk as below, get someone to open up a false seller email correspondence with him and there you go, scam him back with a non existent bike and find his address.
scam the scammer