I tried to make sense of the diagrams and failed first time...
The T6R can do both on/off and OpenTherm so it's working out how to get it to switch that boiler on/off
However...
based on what the diagram for the current thermostat shows, there's
1 240v input that's going to a switch - Red, in one state it outputs on 4 on the other it powers the 2 useful outputs
And 2 outputs,
one straight 240v to the heating load 10(3)A - Yellow
and one via the resistor neutral - Blue
Having a snoop on other discussions on T40s seems they work with a programmer, which is also replaced by the T4
https://www.diynot.com/diy/threads/honeywell-t40-wiring.359817/?utm_content=cmp-true1 - Live from programmer CH ON terminal
2. Neutral
3. Switched live usually to motorized valve, which turns the boiler on via a microswitch. (I was simplifying things in my last post as there are several ways a boiler can be switched on.)
The cable is just old colour 3 core with separate earth, think this is fairly normal as it's a control cable not a line cable. Fairly sure the wee brother dug out 4 core 3-phase cable (he's laugh at me for that description) when he wired my T6R in.
Following the basic diagram, it'll just turn the whole thing on and off which combi's don't really do as they usually have permanent feed and switched feed. Or at least do now.
So on the boiler I make that.
3 - Red - Live to the programmer and to the thermostat
4 - Yellow - Heat On/Off line
5 - Blue - Neutral apparently
makes me think 4/5 is A/B as it's creating a switch with the difference between neutral and line.
Vaillants destructions as to what 3/4/5 are is shit though