When a hot water system is changed from vented to unvented the local authority have to be notified. No G3-qualified person is going to do this work so that puts the responsibility on the householder to notify the LA and pay for them to inspect and approve the work. Failing to notify is a criminal offence.
IMO, the chance of an LA inspector approving your proposed installation is zero. My guess is they'll take one look declare the building dangerous, so it can't be occupied, until the work is remedied by a qualified plumber using the proper components.
IMO, the chance of an LA inspector approving your proposed installation is zero. My guess is they'll take one look declare the building dangerous, so it can't be occupied, until the work is remedied by a qualified plumber using the proper components.