Some of the species of bat found in the UK may sometimes roost in loft spaces. They don't chew or cause any serious damage but their very sticky urine can obviously be messy and large colonies can be noisy. Concerns about diseases are unnecessary as long as you do not handle the bats.

Bats are amongst the most highly protected of Britain's wildlife and it is illegal to disturb them or their roosts, block access to their roost, injure or kill them or set any traps to capture them. It is also illegal to handle them unless to seek treatment for an injury or illness. If you have bats in your loft then The Bat Conservation Trust is a good source of advice. Before taking any action including repair or maintenance work to the area you will need to contact your Statutory Nature Conservation Organisation

Natural England
The Countryside Council for Wales
Scottish Natural Heritage
Northern Ireland Environment Agency