Ok the ones I fitted customers said they were good .(n w london areas) but what they cost you be better off with softner probably same price. 22mm was £500 ish.
Hi I think the supply to face plate is feed from terminal 2 off boiler is there power there also why would someone use earth wire and sleeve to link live and common.
should be a motorised valve with unvented system can you trace where the wires go(.mind you looking at the pressure relief from boiler might be a bad install)