Unlikely to be squirrels, you will normally hear squirrels in your loft in the very early hours of the morning, if it is squirrels, under no circumstances block the hole up, if you do, the squirrel will chew it way in or out and can cause immense damage doing so, believe me I have seen it, either get it trapped or shot!!!!
My guess would be rats or mice, take a torch into the loft, don't be afraid of anything attacking you, that just ain't going to happen, look for for droppings, mouse droppings are tiny hard little pellets about 3mm long rats droppings are much bigger, up to 15 mm long.
Do not use Rentokil!! they charge extortionate fees and you can call in an independent pest control company for far less and get a very professional service. Rats are the least liked but far easier to get rid of!!
Good Luck!!!!