System Boiler (Y-plan) with Google Nest 3rd Gen and OpenTherm?

[Basher]

Do you think it would be possible to replace this Honeywell controller with a fused switch

This is really a question of how the fixed wiring in your property is configured.
If the supply to your heating system is a spur from a ring, it absolutely needs an FCU on that spur before being connected to the heating components.
If you have a dedicated radial from the CU to the heating, there should at least be a switch in the vicinity, but again better a FCU, fused to protect the cables in the heating system.

So the answer is "yes"!

You might ask your heating engineer what was the reason for leaving the Honeywell in position?

 

[Basher]

Sorry, you'd have to have some way of detecting the HW temp too. I'm not sure how Nest operates with this? Evohome measures the tank temp.

Nest has a binary hot/cold input from the thermostat, not a temp input from a thermocouple.
(I have Evohome )

 

[Basher]

Haha just when I had resigned myself to the idea that I won't be able to get it to work

As you see I'm no expert on this, but I still don't understand why you can't follow both the Y-plan and the Opentherm schematics, but disconnect the (switched) feed from the cylinder stat and valve to boiler SL, make the boiler SL permanently Live, and then get Opentherm to control the boiler. Or maybe Opentherm can't turn the boiler on and off?

 

[EvilDrPorkChop]

Nest has a binary hot/cold input from the thermostat, not a temp input from a thermocouple.
(I have Evohome )

Ah, see that would work. I didn’t realise you could do this with Nest. Open them should then just control the boiler.

 

[hometech]

I'm inclined to adopt the occam's razor principle.
Your "engineer" does not know why it works but it does
so don't touch it just in case it stops working