Mould on ceiling. Looks like a leak

[IncessantNameChanger]

What type of trade do I need to see about this? It's in the corner of a detached room. Think the gutter might be blocked. Dh has looked in loft and the roof, is a trades person but sees nothing to cause it

Is this on an external wall? Does the wall outside look wet? When did you last get the gutters cleaned? ( I do mine yearly).

It's possible it's where there is no insulation. Around the edges can often get mould due to the insulation not meeting the edges.

I will have a look at the outside wall. Dh said he had to pull back the insulation but it didn't feel wet, just damp. He did say the gutter had stuff in it but not visibly loose or displaced tiles so I'm hoping it's that.

Clean out the gutter, check it for leaks, treat the mould then see if it comes back

Get some chalk on the wall - you’ll see if it’s damp.

I had a similar thing in my external corner, went into loft and couldn’t see any sign of a leak- top side of ceiling plasterboard was dry, as was all the insulation. I’m putting it down to condensation damp, it’s been particularly bad for moisture buildup in my house this winter. Have been leaving the nearest window open for half hour a day, mopping up off the windows every morning and it seems to be easing.