Here in America we had a nasty case of bats in our belfry a few years ago. After calling several exterminators who said they "don't do bats", we found one who said he would fix the problem.

Since it's illegal to kill the little buggers, he hung up some bird netting over the openings where they went in and out. He only attached it at the top so it hung down rather loosely over the openings. This allowed the bats to go out at dusk with no trouble but when they returned, their "radar" told them that the bird netting was solid and so they couldn't get back in. After about two weeks, he removed the netting and filled the openings with steel wool and, voile, no more bats.

Is that legal in the UK?

Bev and Jerry