Most likely mice and not "longtails" (we Manx don't use the "R" word
) if all they do is scamper about. Longtails will make a lot
of noise, and then they get to chewing on things like pipes and cables........
Remove a spotlight or two and have a look with a torch and a mirror to see if you can see any droppings. If you can and they're small (mouse sized, not 8-12mm long) then just stick a bit of poison up there and it should sort them out - you could do the same for longtails but you don't really want one of those dying in there, especially near a heat source (lights, pipes).
Try and figure out how and where they're getting in and whether it's just between a pair of joists or can they get everywhere? Give Rentokil a shout if you don't want to deal with longtails yourself.