I think the DPC is likely to be directly above the air brick so the mossy brick is below the DPC and not a problem as such
That's what I was thinking too (not that I know more by any stretch - I just spend my time trying to fix damp problems).
What does the door frame look like on the side that has the damp?
If that's damp around the light switch, it suggests that water is running down the wire from somewhere - which means that it's possible travelling along a joist from somewhere else.
Yeah the DPC is above the damp but I wonder if it could have been breached/damaged.
I hadn't thougt about it travelling from elsewhere. We had a new roof last summer so perhaps they messed that up somewhere.
The paint all down the doorframe is darker due to moisture.
That's what I was thinking too (not that I know more by any stretch - I just spend my time trying to fix damp problems).
What does the door frame look like on the side that has the damp?
If that's damp around the light switch, it suggests that water is running down the wire from somewhere - which means that it's possible travelling along a joist from somewhere else.