The Aminopyralid sometimes found in compost can be devastating for your crops. It is from a selective weedkiller used on grasses so may be in lawn weedkiller. It can get into compost via the hay fed to horses.
I have read somewhere that it can take up to two years for it to disappear from your soil.
As much care as possible is taken to ensure that organic material delivered to Humber Allotments is free from such chemicals.