Replacing existing Honeywell wired thermostat and hot water (Y plan) with Heatmiser smart control and possible wireless thermostat

[RandomGrinch]

So I am back to the original question on how to wire it please.

Now this does involve delving into the wiring centre!
They don't want to make it easy for you

Taking the thermostat first, and assuming that you wish to mount the new stat where the old one is and reuse its wiring...

On the neoStat V2, connect:
Brown to L
Blue to N
Safely isolate the green/yellow earth wire in a separate terminal block (there doesn't appear to be an earth terminal on the neoStat)
Black to A2
And a link wire between L and A1

In the wiring centre, disconnect the wire from terminal 4 (probably, as we haven't seen inside your wiring centre yet!) going to the ST6400C (black wire?)
And replace it with a link wire between L (Terminal 1) and terminal 4.

 

[RandomGrinch]

And for the neoStat-HW V2.
Assuming you are going to mount the HW where the existing ST6400C is and reuse its wiring...

You can connect:
Brown to L
Blue to N
Green/Yellow (from terminal 1) to NC (and sleeve it brown)
Black (from terminal 3) to NO
And add a link wire between L and COM

If you are feeling brave, we now have a spare black wire, that we can use instead of the earth wire:
Black (from terminal 4) to NC
And in the wiring centre, connect the spare black wire to terminal 7.
The earth can then be utilised for its proper purpose!

I hope that makes sense!

 

[Samsg30]

Now this does involve delving into the wiring centre!
They don't want to make it easy for you

Taking the thermostat first, and assuming that you wish to mount the new stat where the old one is and reuse its wiring...

On the neoStat V2, connect:
Brown to L
Blue to N
Safely isolate the green/yellow earth wire in a separate terminal block (there doesn't appear to be an earth terminal on the neoStat)
Black to A2
And a link wire between L and A1

View attachment 313135

In the wiring centre, disconnect the wire from terminal 4 (probably, as we haven't seen inside your wiring centre yet!) going to the ST6400C (black wire?)
And replace it with a link wire between L (Terminal 1) and terminal 4.

View attachment 313136

Thanks,

Attached is the picture of wiring center. It's in the loft so I can only make a good guess by looking at the wiring thickness that the room thermostat wire is slightly thinner than the others. Brown wire from Thermostat terminal 1 is already attached to terminal 4 in wiring center and black wire from thermostat terminal 3 is attached to wiring center terminal 5 as already shown in your diagram.
So does that mean only change in wiring centrr is to link terminal 1 with terminal 4 so it becomes permanent live?

 

[RandomGrinch]

So does that mean only change in wiring centrr is to link terminal 1 with terminal 4 so it becomes permanent live?

Yes, and as shown in my diagram..

disconnect the wire from terminal 4 .... going to the ST6400C (black wire)

...and safely isolate it in a spare terminal.