Wiring a Nest Thermostat E where current thermo feeds 2 controllers

Moderator, please don’t simply close this question if the other question asked wasn’t the same, and the answers weren’t applicable. That was sort of thoughtless. Not helpful.

Thank you in advance – anyone who can help. I’ve just moved to a new home. I have a Nest Heat Link E that I had installed in my previous house.

It was pretty straight forward there.

This new house has a single Bosch boiler with programmable controller (time settings for on/off) built in, just like my previous house. However this house also has a secondary programmable control unit (upstairs) where there is a hot water distribution unit. Here is what I’ve found… if I put the main programmable unit (at the boiler) “on” continuously, and set the upstairs one on Auto (programmed) the heating will only come on when the upstairs unit is “on” and the thermostat is calling for heat.

Here is the wiring diagram and pictures of the thermostat I’d like to remove; replacing it with the Heat Link E.