Keeping insects out

pieroabcd

If you are at the ground floow something will inevitably come from the garden doors.
You could consider a mosquito net with a frame that can slide horizontally to put before the window (indoor part).
so even if the door is open the insects will be locked out, like https://www.premier-env.co.uk/horizontal-sliding-fly-screens/

Or something like this, that seems easier and cheaper (but maybe it's more appropriate for a half height window)
https://encrypted-tbn1.gstatic.com/shopping?q=tbn:ANd9GcQUCm_Z4p4r1a4xpug09WKri_Tb1XMH1cG7egYLVc2wpDTRJ8dudPJj7NlqsNfu1R5Qi7WQU7J_VfBh8K3T_sUVpfhqk812zbUsT0umqawU

Savvy_Sue

Slugs are the worst, when they get inside. And you follow their trails, thinking that will lead to their lair, and !!!!!!! trail ends, no slug. It's a sign of damp getting in somewhere. One of my siblings used to be very plagued by them, you'd come down and find them drinking the cats' milk! Shuddering just thinking about it! Anyway, our problems have been much improved by replacing the old (and very rotten) back door. 

Yes, you can get repairs to PVC doors and windows, and I'd start with the doors. 

I had to get DH to evict a snail from the hall ceiling the other day! 

Please don't kill off the wildlife in the garden though. The birds and the bees will thank you for it. 

Mnoee

I think the issue with filling every crack even a woodlouse could fit in is that you'll remove any ventilation too and potentially end up with damp issues. Make sure you're still opening windows and whatnot using screens if you have to. 

One of my cats is a fantastic bug hunter - happily eats them too. Not for everyone (nor every cat, the other shows no interest in creepy crawlies), but certainly can be an effective solution.