How big is the lawn? What sort of Atco is it and what do you mean by it not coping? The fact that you considered a sit-on mower suggests it can't be very steep; what is he gradient?
Sounds like the tree was there before the houses.
I'm surprised you don't see the thrush singing, ps2. They usually stand on a very prominent perch.
It's really funny watching it with the sound turned off.
There seem to be a lot of second nests this year, so please be careful about doing hedges for a while.
Aspect can make several weeks' difference.
You have to be very careful, I agree. I'm still having trouble identifying from the picture - looks a bit like alkanet or comfrey, in which case a bit of short-term yellowing will be worthwhile!
I'd paint the leaves with glyphosate, mow the lawn normally and wait for results, although it might take several summers.
Does the farmer even know what his beasts are doing? Just have a quiet word with him about your garden, and try to find a non-threatening way of introducing a timescale into the conversation.