"One problem with getting manure from farms and/or stables is that even if they haven't used the dreaded herbicide on their land they may have brought in hay or silage of unknown provenance...."

Yes, but armed with that knowledge you can ask the question. I don't think a farmer is likely to lie about the provenance of their bough-in food just to close a sale on some manure!

Of course those that don't know to ask, and farmers who are unaware that the fodder they used was contaminated, are, sadly, at risk.

"As I said, "Levingtons" have confirmed, in writing, that their stuff is NOT contaminated"

I'm happy with that, but I'm not, persianlly, reling on it. The supply chain has the risks of potentially lots of small suppliers, and imperfect relay of information about herbices that are used. I have no idea whether this is actually a Levington problem, but:

"We have the aminopyralid problem and definitely from Levingtons Organic Blend Farmyard Manure" - Allotments-uk Forum - Contaminated Horse Manure