uPVC window drain holes

Wire wool should keep vermin out, but allow water through. Can you get stainless steel wire wool?

I imagine water build up was due to the holes being blocked.

Yes, but it's a lot coarser than "wire wool", which is be a good thing against sharp teeth and an ability to pull things apart.

You can get a copper mesh for rodent protection...

Dave

Not answering your question, but cheeselogs? Never heard the term. I had to look it up. Ah, you mean slaters!

The holes usually have a cover that snaps on - this should restrict driving rain, and make the holes look a little tidier.

The holes lead to what should be a sealed area between the glazed unit, the frame, and the retaining trim. There shouldn't be any way for water to get past the trim (silicone strip on the outside, plastic inside) - at least in the quantities you're seeing. And certainly not enough space for an insect.

So I suspect your problem is not the drain holes, but the fit between the opening windows, sliding panes, and the frame.

Would have thought copper was more chewable than steel?

My experience is that its mouse-proof and in this case wasn't the issue even smaller creatures ....

Dave

We had a door fitted a while back but the fitter was missing the covers at the time. He said he would drop some off, but never did. So since we had some existing windows also missing covers, I drew them up in FreeCAD and 3D printed some. Worked rather well printed in white PETG. So if anyone wants the model or a sliced file let me know...