I would recommend you install a camera connected to a video recorder (recording continuously), or better still a DVR. You could either install it covertly, to catch your neighbours in the act, or overtly, such that they know they are on camera (when in your garden).
From a 2nd floor window, it should be fairly easy to position the camera such that your whole garden is covered. The image quality need only be good enough to convince the police that it is your neighbours who are entering your garden without good reason, for them to give them a formal warning.
I would recommend a low light camera, since your neighbours may take to nocturnal damage of your property. I have a security cctv camera which has a specified operating light level of 0.03 lux. I would estimate its sensitivity to be around that of the human eye. Most cctv cameras require around 1 lux of light, once the sun goes down, additional lighting is required.