(I feel like Dr. Who now) A few years ago there was a piece on Gardener's Question Time about horse manure that was killing people's gardens. It was caused by residues of weed killer (presumably for killing ragwort) that had passed through the horse's digestive system.
I don't know much about weed killers, but would NEVER personally put plants treated with it on the compost heap, just to be on the safe side. When I use 'Weed and Feed' on the lawn, the packet says not to compost the first 2 or 3 cuts of grass. I know some weedkillers are neutralised on contact with soil, but I still would not feel happy about this.
Have just looked through my notes on what one can and cannot compost; there is no mention of weedkiller. So am going to email Garden Organic and ask them.
Have just sent them a message, hopefully they will get back to me after the Bank Holiday
weeds treated with glyphosphate can be composted. adding other treated materials can actually stop the compost process. lawn weed and feeds and systemic treatments seem to be the worst-they are meant to linger but other weed killers are very long lived so probably better not to use any in the compost.
That seems definitive. Thank you Artjak and David.
Bumping this as Garden Organic have emailed me to say they will answer query asap