I'd be interested to hear what the RSPCA have to say on the subject. It is likely to be either a fox (I once left my cats lead in the garden overnight and found it the next day totally shredded by foxes which did make me think how much damage they could do to something alive) or worst case scenario it could be one of those "status dogs" who are trained by their owners by being given animals to attack, a large dog could inflict damage of this kind quite easily. However as it was in this gentleman's garden (which is presumably private and not easy to access by people) it seems less likely to be a dog and more likely a rogue fox - they are sadly growing bolder and more agressive. Keeping pets in after dark is probably the safest bet and be especially watchful of older more vulnerable pets who are less able to fight back.