So it turns out that the process at the time of writing is unpublished on the website and requires an email to be sent. You cannot make up your own any more as the first part of the number relates to the issuing agency.
Email your name, address, details of your build, and request a VIN number to dvlavin@dvla.gov.uk and in a few weeks the forms turn up in the post. This form has a place for a garage to stamp to verify that number has been put on the chassis correctly, and this is used later in the process of the SVA test.