Hi Martdog, I'm so sorry to hear of the loss of your hens, that must be very distressing and worrying. I'm afraid the only answer, in a small urban garden, is a really secure run, with a roof or a top made of weld mesh as foxes can climb, strong weld mesh sides, and either a weld mesh floor attached all round to the sides to make a sort of box, or buried deep into the ground all round to prevent digging. Electric fencing is also effective, of course, but probably not suitable for your garden. If you can have a solid roof on top of the mesh, eg clear corrugated plastic, the run will stay dry, no mud, and you will be amazed at what a difference this makes to the comfort of the hens, and for you when you come to clean it out.
If the run is large enough, ie at least 2 square metres floor area per hen, they will be quite happy in there if you can provide a nice dust bath, a perch for them to hang out on to see what's going on, somewhere shady for sunny days and somewhere out of the wind and rain when it's nasty weather. You can give them some greens every day and otherwise they will do well on a simple diet of layers pellets and a small handful of mixed corn as a treat in the afternoon before roosting. I agree with you that small runs, especially those fixed to coops, are quite inadequate and do cause stress, but if you can make them a secure run which fulfils their needs, it will be much better for them, and less stressful for you, than living in fear all the time of another fox attack. Be sure, he will be back, now he knows they're there! Hens are very adaptable, and If they have sufficient run space can be kept successfully in a whole variety of situations.