Thanks for all the help above. On that day we had a plumber in who was up in the loft in the morning and had left the loft hatch open all day so it’s possible it came into the house that way rather than the open window. I’ll check out the loft over the weekend and see if there are any signs up there. I’ll also have a read of that website and give the helpline a call if necessary.
I was planning to put up a bat box (or perhaps a few after reading that they move about depending on the season) in the trees rather than against the house. We live fairly rural so have plenty of trees and space around us. We also have some out buildings which I’ve checked but can’t see any evidence of bats, only a couple of old bird nests.
I’ll report back with any findings.