Totally agree with NOG. Send the details to the Chief Executive of your local council, and give him/her a timescale to reply (7 days?). Keep copies of all correspondence. If you don't get a satisfactory reply then contact your local MP, the newspapers, the National Allotments Association etc. It's not just your crops, although this is bad enough, what are they spraying and does it have any possible consequences for local people or especially children coming into contact with the sprayed areas.