Right fleas
Firstly it depends on the products that you use.
The only good flea products that are really reliable are the sort you get from the vets. You don't have to buy them from the vets - vets UK are cheaper and if you order via one of the charities (links) you will be giving them a donation.
Just because a thing is a spot on don't make it any good.
The same goes for household sprays - I don't think you would find anyone on here that recommends bob martin stuff for example.
Do not consider using a dog flea tratment on a cat, they contain different chemicals and can prove deadly.
Fleas tend to be animal specific so dog fleas prefer dogs, cats cats, people people etc. Your cats could well be bringing them in from outside but if they are all treated , one bite and its goodbye fleas.
Hedgehogs are a good source of fleas and ticks (though I believe the fleas to prefer dogs to cats).
If you're heating has just kicked in then you might have awakened the dormant fleas around your house, they do have a habit of crawling out of the woodwork!