Advice on bay window damp and mysterious hole

[stressedpetal]

I've had damp in my bay window for a couple of years and it's been really difficult sorting it out. Everyone seems to have a different idea of what's causing it, and unfortunately it does seem to be a combination of things. My front yard is tiled too high, my drainage system was ancient in a shocking state (now replaced) so that water was just flooding my front yard and contributing, and a broken gutter meant water was falling on my windowsill and rotting it totally, soaking the plaster.

I thought fixing the drainage would help but the damp and mould is still there, despite dehumidifer, and worse even now with the cold weather. I'm having new windows fitted next week which will sort the rotten window problem, but I'm anxious still about the damp, and now about how the damp will affect the wall under my very expensive new sash windows, having been assured that the new windows will fix the damp (I need new windows desperately though as none have trickle vents and huge condensation issues). I've been going obsessively through photos from when the house was renovated 2/3 years ago and found the attached image, and am wondering what this hole is and whether it is the ultimate culprit? This is where the mould is concentrated in the bay now - in the renovation I think they just plastered over and I assume they didn't fix the hole. My renovation was done by builders whose work I am really unhappy with now a few years on as it wasn't done to a good standard and I am having to patch things up, so I wouldn't have expected them to fix a hole unless I explicitly told them. They replaced rotten joists when they renovated. Could this be it, and what is my best course of action? I can't delay window install as I've paid a hefty amount of the install costs. My plan right now is to get the windows in, keep dehumidifying, investigate under floorboards and hopefully that will let whatever's going on under there dry out a bit, and sort out the too-high tiled yard as a matter of urgency.

Is it solid or cavity wall? What does "tiled too high" mean - can the bottom of the wall outside the bay not dry out?

I wonder if the hole is a symptom rather than the cause - it was saturated for a long time and the plaster collapsed.

Two things spring to mind...

First, are there any airbricks at the front of the property that have been rendered over or covered because the tiles are too high? I had examples of both at the property I'm renovating. If you have suspended floors, there needs to be ventilation underneath via an airbrick as I understand it.

Second, assuming the cowboy builders just patched over and plastered where the hole is, it's likely that the temperature of that bit of wall is colder than the rest of the wall. When you heat the room, condensation is more likely to form there and that will be causing mould.

Thanks for your responses. When I mean tiled too high I mean that the tiles are 'crowding' the external corner of the house if that make sense - and there are airbricks but they are partially blocked by the tiles, they are lower than the tiles. I think this combo might be resulting in water coming into the subfloor as it can just wash right in, but the damp is only bad in one corner, which you can see the wonky tiling of outside. The tiling is a dodgy job by prev owners and not only is a bit too high ventilation wise but also runs water towards the house. I don't know how the damp problem wasn't a huge issue for the previous owners. There was visible damp through the wallpaper when we renovated but it has got much worse. Maybe they did plaster over and now it's colder, @ClematisBlue49 - I hadn't thought of that. It's a solid wall @pancakerobot and hadn't thought of the hole being symptom not cause either.