I assume these are grey squirrels as the pest control people would never poison red squirrels - it's not allowed! And in any case it is very rare for red squirrels to enter loft spaces.
My neighbour traps them with a baited cage.
Then puts the cage and squirrel in the water butt.
Another neighbour shoots them.
They are hugely destructive and will cause damage in the loft.
If you do bait and trap them, remember it is illegal to release them so you will have to destroy them.