Originally the system had Sunvic MOMO valves on both zones and HW.
That would explain why you have a wire connected to the SAT terminal.
Now I have the common Honeywell zone valves which have the open lever, synchronous motors and spring shut. I can't see the model nos and likewise with the thermostat as it has worn off with age!
You don't need the SAT connection on these valves. It should be disconnected
at both ends and made safe.
Terminal 1 is normally Live, 2 = Neutral, 3= Call, 4 = Sat.
You can easily confirm this with a meter as follows:
The voltage between 1 and 2 should stay at 230V, for all positions of the stat.
The voltage between 2 and 3 should be 230V when the stat is turned to HIGH
The voltage between 2 and 4 should be 230V when the stat is turned to LOW
If this checks out, the terminal numbers are as above.
This checked out so I disconnected wires 2 and 4 and the stat still operated the valves and boiler.
The new stat does not need a neutral connection, so this should also be disconnected at
both ends and made safe.
Connect the new Stat as follows:
Live (wire originally to terminal 1) to A terminal
Call (wire originally to terminal 3) to B terminal