Jon Rouse wrote:

You've done better than us then, when I finally tracked down the
correct department in the council they sent out someone from the Parks
department who looked at our brown hedge, and told me that
glyphosphate wasn't a systemic weedkiller and as the wood was green
under the bark it would all grow back. Of course it didn't, and as we
have no idea who farms the land (probably some overseas
multinational) we have no way of taking it further.

Yes you do! If it's a farmer who's done the spraying, s/he has to comply
with the rules and these are overseen by the HSE (Health & Safety
Executive). Contact your local department and tell them what's happened.
Take pictures and video, if possible. You can find out who owns the land
through the Land Registry but the HSE will be able to find this out through
DEFRA anyway. However, regardless of who owns the land, it's the person
doing the spraying who's liable.

Jaqy