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  • condensation would only form in that shape if it has a corresponding cold bridge, is there anything like that in the wall or the other side?
    Looks like it’s travelling in that way inside the wall rather than condensation so a slow leak from something, presume there’s no diagonal crack in the brickwork?

  • There is render on the outside wall. It’s a solid wall, nothing on the other side/inside. Render looks OK but may get a ladder to take a closer look

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