Sorry to hear of your situation monty.
I would say that the first port of call would be to contact NSALG asap to ask their advice on the matter. They may be able to advise on whether they think soil samples should be taken on your plot prior to and after treatments in order to prove contamination, etc.
Nonetheless, I would also refer this matter, clearly citing your concerns and referencing your sources, to your local Councillor asap, cc'ing the Environmental Health and Allotments teams as a matter of course. I think you should also mention that you are currently seeking advice from NSALG.
In this correspondence, I would request clarification and assurance of the following points in writing:
-What exactly are the chemicals being used.
-In view of the advice given to the plotholder (why would you need to have a growing ban of 18 months if it was a Glyphosate based spray?
), what assurances can be offered as to the safety of crops on the adjoining plots during the spraying process (ie, do they intend to screen off adjoining plots, etc).
- What assurances can be offered on concerns of contamination risk to rainwater runoff onto other plots (obv. if Glyphosate - fine but if it is a combination you need clarification).
-A copy of the method statement, including health and safety and risk assessment the chosen contractor is utilising for the job.
-How any risks to your plot are going to be mitigated and compensation issued should any soil samples (if you wish to go down this route) be found to show contamination.
-A halt to any works proceeding until the above has been confirmed.
-A copy of the planned spraying schedule so you are aware of them in future and the name of an officer that you can liaise with to discuss the treatment.
Hopefully, as rowan57 states, these SHOULD pose no risk to your plot if applied correctly. However, you need assurances in writing that this is going to be the case. Surely as a leaseholder, you should at least be provided with this as a minimum.
Should you get nowhere, I would find out the publication cutoff for your local paper and get onto them pronto.
Like Rachel (Mdme Cholet), I would be unhappy finding this out.