Crumbling plaster and damp issues

Hi,

I would like some suggestions on what could be causing this issue on an extension wall built around 18 years ago - it has deteriorated after time but worsened over the past 3 years. Most of the black spot mould has just been cleaned in the photos, but it’s all over the walls pictured and the plaster has crumbled showing the brickwork on a patch of the wall. A tradesman suggested that the door needs to be resealed as there’s gaps, could this cause such damage?

Any help would be much appreciated, thanks

 

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