They are not trespassing on your property as they surely have a right of access to the rear of that property. I live in a block of 4 terraces in one of the middle ones. My right of access is to walk around the back of my neighbours property to get to my own
I wouldn't agree with chaining the gate as this restricts their access. Imagine there was a fire and they couldn't get out. I I reckon you'd have a lawsuit on your hands
I appreciate the problems you are having but I think you need to speak to the people they rent it from.