You have my sympathy, but I disagree with your statement "but in my opinion they should go a lot further to compensate the many allotment holders who have been devastated by this chemical disaster."
The farmer should never have sold the hay to the stables, and the stables should have become aware from the press (at some time in the last three years) that there was a potential problem that they should be careful about. It was reported in Farmers Weekly in November 2007 (and probably at other times since then too)
As you have found out that the grass was treated with Forefront then someone you have spoken to must know that that product specifically was used, and they must therefore have seen the label when the chemical was used. That label, at that time, was absolutely clear about the side effects and the responsibility of farmers to inform recipients of hay, silage, manure, slurry etc.
I posted the instructions, verbatim, from the label in another forum in 2008. Here's a copy:
"Manure and Slurry Management
Do not use animal waste (e.g. manure or slurry) from animals fed on grass treated with Forefront, of fodder resulting from grass treated with Forefront, on susceptible crops e.g. peas, beans and other legumes, sugar beet, carrots and umbelliferae, potatoes and tomatoes, lettuce and other compositae, or land intended for growing such crops.
If grass, hay, silage, manure or slurry is exported off your farm, it is your responsibility to inform the recipient of this information"
(the boldfacing is mine)
The farm has had three years (during which time knowledge of the issue in the farming industry has become increasingly commonplace) to make the stables aware of the potential problem ...
As others have said, Dow has obtained a licence for this product under current regulations, and whilst you may not like the product DOW has done nothing wrong. I'm sure you won't see it this way, but you could take the view that Dow is being generous in offering to take the contaminated manure away; the responsibility for your problem lies solely with the slapdash husbandry of the farmer or contractor who used the chemical in the first place and flagrantly ignored the hazard label on the product.
I'm not pro-Dow or pro Big-Pharma. I think the farming community was remarkably slapdash with their use of chemicals in this instance, and the result was widespread distribution of contaminated manure. That caused the product to be temporarily withdrawn. I am not optimistic about the so-called tighter notification that was introduced when the product was re-licensed, and always seek reassurance from the farmers I get my manure from as to the provenance of the hay used to feed the animals.