You need to be throwing all your resources at this one. Ringfence your allotment with 20ft high concrete wall, proper steel entrance door with retina scan entry system. Hire some goons and install watchtowers in each corner. Lazer systen like in Mission Impossible that sets off massive alarms and big flashing red lights, lots of CCTV, and a direct line to GCHQ. Also, put a big concrete roof on and keep the Sun and Sky inside your allotment. That’ll show the Bastards who is Boss!
There are a few on my allotment, who if they had the funds would consider most of the above 😉
The following words seem to bounce about quite often on my site:
mantraps
Razor wire
advanced security cameras
electrical shock devices
paint traps
dirty bombs, not radioactive, but I am sure if they could get hold of.
evening patrols with improvised weapons. 😯
motion activated traps that would maim at the least
microdots
trip wires