Wiring switching from Nest to Hive

[emmeth92]

Sure, here is the wiring below.

Just to note I had no connector blocks handy so black wire is connected to earth terminal, as there was no connectable earth wire.

Also since switching grey and black then back again there's now no power even in this wiring set up (which at least was giving power to the receiver last night - maybe I have to reset the hub or something first?)

 

[RandomGrinch]

How have you been isolating the boiler when you've been working on this wiring?
There looks to be a fused connection unit, to the side of the Hive, does this control the boiler and have you been removing the fuse when you have been working on it?

What you describe sounds like a fuse has blown (if an RCD, or RCB hasn't tripped on your consumer unit).
This may happen, if there had been a short circuit.
And that shouldn't have happened if we are using the live switching terminals provided on that boiler.

Can you check that the fuse is working and that nothing has tripped on the CU?