+1

Since your garden is surrounded by other gardens plus a few fences, then the foxes very likely think that you (and your neighbours) have installed a Centre Parc just for them; there may be earths under more than one garden in the terrace.

The sonic deterrents do seem to work. Friends had rented a cottage that was beautiful but had Mus musculus in situ as well, and three or four of the sonic thingys made the little houseguests bail quite fast.

But city-smart foxes might carry earplugs. There are foxes, coyotes, skunks and raccoons in the gardens of the houses and blocks of flats in my neighbourhood (Vancouver's West End) and they've settled in for the long run.

You may have to find some way of creating detente with the foxes!