I'm sure they can get from loft to loft. Even though there is almost certainly a party wall. the bricks will have been snapped to make a slope somewhat close to the roof timbers, but not a rat-proof seal.
They need constant water, so verify there are no tanks without close-fitting lids, and no leaks, in your own loft at least.
Put down lots and lots of little dishes of bait, in the loft, behind the cooker, under and inside all the kitchen units, check them, and refill them every time they are empty. I use bright red plastic flowerpot saucers. When they stay full your problem is gone.
Wash all packets of food, and put them in metal biscuit tins if you can, but plastic boxes will do.
Clear away pet food between meals and sweep up spillages.
If you or your neighbours have a pond or other water feature, get rid or surround with weatherproof bait-boxes.
If you or your neighbours have a rat-table bird table which is frequently replenished with delicious rat-food bird food, it will attract them.
If you or your neighbours have decking or a shed, it provides a safe and comfortable place underneath to bring up an extended family of rats.