I'm presenting this as fairly as I can. Once I've seen some responses I'll let you know if I'm Person A or Person B! It's long and probably quite petty but bear with me...
Person A and Person B live in side by side detached houses. In front of the houses is a square of garden. Each house owns exactly half of the square.
Person A moved into the house last year. On the date of moving in, there is a large, empty, circular flower bed in the middle of the shared garden. It crosses the boundary between the gardens.
Person B pops round to Person A's house within a week of Person A moving in to say that the flower bed was going to have a weeping cherry planted in it and that the previous owners of Person A's house had agreed to this, as it would look nice. Person B wants to know if Person A has an issue with it. Person A says no, that's fine.
Tree duly planted, small border of bricks, flowers under tree in bed. All good. Person A and Person B have no formal agreement but more or less share any minor gardening required (e.g. weeding). Neighbourly harmony is free flowing.
However... the houses are on an estate with a factor who does occasional inspections. Following a recent inspection Person A and Person B receive a notice from the factor addressed to 'the owner of the plot with with tree' informing them that it breaches the conditions of the estate and that there will be a £250 fine plus tree needs removed.
Person B visits Person A and suggests the fine is split down the middle. Their reasons: both parties agreed to tree, both have maintained it, both have benefitted from it looking pretty in the front garden and adding to the curb appeal of the properties.
Person A refuses. Their reason: they only agreed to be polite, it is very much Person B's tree which just happens to be on a bit of their garden, they took no responsibility for it when saying it could be planted.
So... who is right here?!