Hello Neil
If you haven't any certification, then the Boiler Tester will need to inspect the bare boiler and treat it as "new and untested". If you can prove it's been built before the CE marking rule came into place (I think it was 2002) then it won't matter if it was built commercially or as an amateur build. Technically if it was post 2002 and commercially built, it MUST have the CE mark or it cannot be tested!
The Boiler Inspector should be able to examine the boiler easily – on a Clayton it's all pretty well visible to check for correct penetration of brazing – and then give it a 2x hydraulic test. Once it's back on the chassis with all the fittings in place, it can be tested to 1.5xWP and then given a steam test.
If Sunderland are in the Northern Fed, it's likely that they will issue a new boiler ID to be stamped on it for future re-certification.
Good luck!