Crack above window - damp from old vent?

[Freshbread]

Hello all,

In the last week a horizontal crack appeared above my kitchen window, moving outwards from a plastic vent cover in the middle. I removed the vent cover and found the cavity to be full of fibre wadding which was quite wet and rotten, so I pulled the immediately accessible stuff out and cleaned things out as best as I could. Just beneath the crack is what I would guess is a metal edge bar.

Looking outside, the wall has a rough render allover and there was no hole for a vent (blocked). There is a timber frame structure secured to the wall above the window and I found a <1 mm crack running horizontally across the top of the window which I suppose is the where moisture has come from.

As we were about to travel to the in-laws for Christmas I quickly sanded the external crack and slapped some polyfilla in it to prevent more moisture getting in whilst we were away, but I know this is a very temporary fix.

Any advice gladly received!

 

[Freshbread]

Doesn't seem like anybody has an easy solution for this, which trade should I be calling?

 

[John D v2.0]

If the insulation is sodden then there is reason coming through the outer leaf (normal if it's not rendered) and not hitting a cavity tray above the window. Presumably the lintel is sitting there wet and has rusted now.
Ideal construction would be new cavity lintel with a tray over and weep vents.

 

[Freshbread]

It's foam that was used to fill the blocked off vent rather than cavity insulation as far as I can tell. I'm not sure what the lintel situation is, the vent sits above what seems like just a timber that is about 20 mm thick, so I don't know if the proper lintel is above the vent?

 

[John D v2.0]

Either way it shouldn't have water sitting in the wall, anything should drain outside and dry out pretty quickly. I guess the current situation is balanced the other way and needs investigating and resolving.

 

[Freshbread]

This is the view inside the vent now I've cleared out the fluff/rubble that was filling it. Metal beam looks damp and a little corroded. So what do I do? Call a builder?