Central Heating Thermostat Questions

[theaxe]

I'm attempting to replace the thermostat in my brother's house (to a NEST). Looking at the existing setup I see:

1. A timer/controller next to the boiler. This just seems to turn the boiler on / off at set times. I assume the hot water then flows into the tank and that it turns off if a) the timer turns it off or b) the return temperature hits some limit.
2. A timer/controller next to the tank. It seems to be wired so that if the controller is ON then power goes to both of a) a thermostat on the tank and b) a room thermostat. Either of these then send power to the pump (the wall thermostat if the room is too cold, the tank thermostat if the tank is above 60 degrees).

This seems unusual to me, I can't see how the upstairs setup can trigger the boiler. I also don't understand why the tank being above 60 degrees would trigger the radiator pump separate to the room thermostat. Maybe they should be in series but they're in parallel.

If anyone is able to explain what's happening here I'd really appreciate it. My current thinking is that I should be able to use the NEST to replace the wall thermostat (to drive the pump) and leave the rest as-is.

[theaxe]

Thanks for the reply. As far as I can tell the timer by the boiler is for hot water and the one by the tank is for the heating. There is an immersion heater but that's just on a switched spur and is generally off.