I don't know the answer, but I would have thought that you would be safe as long as your veg are washed or peeled before eating. I don't think other plants would take up the toxin, but I can't be sure. Would it not be possible to move your water butt?

Thanks for that. I can't move the butt so I will probably have to get a better fitting lid because I do not want to cut the Yew down. I did wonder if the toxin would be absorbed by the vegetables in which case washing them would be ineffective.

Any toxins in the water, and I doubt if there would be any, will not transfer into other plants. Whilst I would not recommend actually drinking the water your vegetables are quite safe to eat.

Agree perfectly safe to eat.

Thanks everyone.