Grrrrr! I agree, bastards! I can´t stand such parasitic creatures. And they are in plenty here too...everywhere.
I am sorry, I know how much you care for your garden.
We used to have similar allotments here but they changed into fenced areas with tiny cottages and vegetable and flower and fruit plants around. Sometimes it is funny because the owners want to use their small area (4x4?) for building as tall structure as possible. The law probably doesn´t limit the hight of these. And believe me, some look silly enough!