The white wire is terminal 5. You said terminal 4... so I did terminal 4 - see picture 1?
I guess you meant N+4, then N+5, then N+6?
If I knew what I was doing I wouldn't be asking chap.
Give me 2 mins and I will update.
There is some wiring behind the controller which includes a loose connection (third picture attached). This might be the issue as I could have knocked it. The grey wire from the boiler I guess is a live return(?) and connects to the red control wire which I think goes to the valve(?) via the circuit box. The yellow seems deliberately blanked off and appears blanked in the circuit box (top right). The light blue looks like it might have been attached to something behind the controller. In the circuit box I think this joins another light blue in junction 2 - which seems to connect to a dark blue wire from the valve and a black wire from another cable (which seems to be a mains type cable).
Assuming I’ve done it correctly for now. Measuring it with the HW/CH on/off doesn’t seem to make a difference.
Doesn’t sound safe to have a live wire knocking about in the junction box but that aside could it be the issue? The only thing it could reasonably connect to in there is the grey wire from the boiler (the live return?) as it wasn’t connected to the Altech previously.
The neutral at the nest comes out of the boiler (with live, earth and a grey wire I guess is live return. This last one goes to a red wire in the circuit box).
The light blue wire in the old room thermostat box is live but in a terminal block isolated off.
There is another cable joining the circuit box which has a black wire going to the same terminal (2) but I don’t know where that comes from. it might join the conservatory system since it runs off in a different direction but I didn’t think that system touched this one.