That’s what bug bombs and exterminators are for.
And glue traps!
I agree, bug-bombing the place would work. Maybe they don’t have bug bombs in the UK, tho?
And btw, those do not look like spider bites. At all.
My brother is an expert on pest control. He was telling me this past weekend that glue traps are the ultimate, because spiders are cannibalistic. If one spider gets trapped in the glue, it attracts others like a magnet.
He said to lay the traps out along the baseboard on a wall - maybe under a bed. He told me it will catch spiders in even the newest of homes.
“That’s what bug bombs and exterminators are for.
And glue traps!”
And blowtorches => https://www.youtube.com/watch?v=1A_mWBX1gRs