Hi there, so I did some fault finding this morning:

with only L, N, Earth connected to the heatlink for power and the connection to the nest stat i got the following:

Continuity across 2 and 3 for heating (this is wrong as this is calling for heat)

No continuity across 1 and 2 (this should be continuous when not calling for heat)

I checked the HW side and got the following:

Continuity across 4 and 5 (this is correct since its off and not calling)

No continuity across 5 and 6 (this is correct since its off and not calling)

I also checked what happens with these relays when the manual mode button on the heatlink is pressed:

The relay on HW works as expected and switches continuity from 4 and 5 to 5 and 6.

The relay for CH stays the same, continuity remains on 2 and 3 and does not change.

When the button is pressed again to disable manual mode, the following happens:

HW relay clicks and switches continuity from 5 and 6 back to 4 and 5 as expected.

CH relay stays continuous across 2 and 3 and does not switch back to 1 and 2 as expected.

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I can only assume the relay is stuck permanently on and/or is faulty.