A neighbour has been underdosing her cats and dogs with anti flea treatment, and that has apparently caused a fipronil-resistant strain of fleas.
The worst infested was taken to a vet and got alternative treatment, the vet gave them flyers to hand out locally to other pet owners highlighting the need to change treatment. £67 to treat her pets, £185 to deflea the house.
They have a mangy tomcat who tries to mount anything standing still, male or female, so it's fleas are going to pop up everywhere soon.