To look up a number on the DVLA site go to:
On the left hand side click on ‘Vehicle enquiry’ then click next then enter the registration number and make but don’t use the dropdown list as they don’t always get the maker right (e.g. ‘Layton’ is listed as make ‘Norton Dominator 7’, we filled out the forms correctly!!) so you may need to put the model in as well!
Have a play and see how you get on!