Yea I would zone it, you have full control then. 2 flows and common return
I would use dedicated pump for Ufh
You can get the design guide for about 18 pound
If you link 2&3 to the live it will never turn off will it?
You need to take a 5core from the boiler live neutral and 2 switch wire you'll have earth left over
Just service to mi's as far as testing the flue being ott I think you need to read
Reg 26(9)
(9) Where a person performs work on a gas appliance he shall immediately thereafter examine—
(a)the effectiveness of any flue;
(b)the supply of combustion air;
(c)its operating pressure or...