Damp patch on living room wall

I'd probably suggest checking the cavity around that area, things do build up over time and regardless of a damp course, if its high enough it will bridge it and find a way through.

This condition is probably hygroscopic dampness, yes it looks worse when it rains but it’s actually linked to the humidity not rain specifically. There is a patch of thick plaster where the damp is showing , plaster should not be more than 2.0mm in thickness , ever. It’s probably an old repair, you cold scratch around to find the patch . Then cut it out and replace with sand and cement .

You might have something bridging the damp through to the wall of the wall has previously been boarded with drywall adhesive it will grab onto that and come through.

As you stated you have just had the gable wall repointed. With out more information. I can presume the building is pre 1940s So this would be a solid 9" Wall. If the damp is at low level. Bellow 900mm. Then yes highly likely to be a DPC issue. If at High level. Then this is probably down to poor ventilation.