No heating unless hot water is on
I just added the extension kit to my Tado system, inc. smart rad valves in most rooms. I had a UK backplate on my old control so it was a straight swap, plus some reconfiguration of the smart thermostat following the Tado instructions for a Y-plan setup. Everything appears to work as you'd expect - I have a separate hot water switch in the app, with a schedule, etc.
However, I have no heating if the hot water is off. The app says it's heating, but the rads are cold. If I enable hot water in the app there's a click in the extension kit and bingo - heating. Also, if the hot water is on, the rads WITHOUT smart valves also get hot, which implies that I have an all or nothing type system now, and the hot water is like a master switch.
I looked in the manual for the boiler but can't quite tell how the backplate is connected - and anyway, I've not changed any of that.
My boiler is a Worcester Greenstar 40CDi, with a separate water tank and CH pump.