Neighbour dispute - brick wall damage/stained

[Crooksy]

Hi, our back garden meets with our neighbours back garden and is divided by retaining wall with approx 10ft drop. The two properties are divided with a wooden fence. The fence recently came down during a storm. As it was a shared fence, we spoke to neighbour and aid we would get someone to replace. Workmen fixed fence but while adding postcrete to new fence posts, the ‘mix’ seeped through retaining wall and has now stained their wall. New fence was replaced with little fuss and for very little cost. Neighbour now not paying half the costs and wants their brickwork wall repaired. We are happy for them not to pay anything and simply look at the back of our fence, but I fear they will push for repair. Any advice how we can move forward? Thanks