Its simple, phone your local building control and inquire, they will then give you a control number for a fee. They will inspect and sign off if all is proper and if not charge you the same again to do the same again after whats not correct is rectified. I did the same with an annex I built and saved a good amount of money but I doubt that very much concerning an un-vented cylinder when a registered guy could do it when part of a competent persons scheme for a couple of quid