I have just registered a quad for agricultural use, so i thought i would let you know how i went about it, first go to the link below and decide what applies to you, i went for the full lighting kit including fog and hazard lights made up of parts from ebay.
Then i insured the bike with the NFU TPFT any driver £150 the policy just had the frame number on it,
Ring DVLA and ask for form V55/5 with guidance notes to be sent to you, note that a lot of the questions will not apply to you so just leave them blank.
Fill out the form and send it to your local DVLA office with a cheque for £55 your Passport or Driving licence, Insurance Certificate, and utility bill,
After a few days you will receive a letter asking you to attend the local DVLA office so that they can inspect the Quad
At the inspection they are only interested in checking the frame ,and engine number if it has one, mine did not
A few days later you will receive a form with your registration number on it and your free road tax disc,
take this form to your local motor factor with some identity and get your number plate made up,
Put both number plate and tax disc on the Quad and you are ready to go
The regisration document will turn up in due course
Hope this is of use to someone,