I have not had to do this before but took the first step today to register an Open Enduro I bought from eBay without a V5.
In order to get an age-related registration number the DVLA require a manufacturer's dating letter, and the bike to be MOTd and insured.
I rang Yamaha UK today and got straight through to Moira Dwan in Customer Services (direct number 01932 358042) who was able to tell me from the frame number that mine is a Model 4GY1 manufactured in June 1993 and intended for the Japanese domestic market. Excellent so far!
The downside was that I then had to email my request to cust.service@yamaha-motor.co.uk for a dating letter. It will probably be 3 weeks before someone phones me for credit/debit card details to take the £40 before sending the letter
Plenty of time to finish off the bike and get an MOT then!
The moral is that, if you are going to register a TTR, get that email off quick else you will be waiting....