You may well have Gas Safe people you work with but that certification has nothing to do with working on an unvented hot water system. You will also know that you do not need a certificate for your tenants and your Rental Agents will know that as well.
All in all you really do seem to be confusing very different disciplines which does say a lot about your underlying knowledge.
All in all you really do seem to be confusing very different disciplines which does say a lot about your underlying knowledge.