We have an open plan front garden, but I have just found out that my neighbour has been cutting the grass about 3 meters on my side. Can I erect a fence were the deeds show it, even if he has cut the grass there?
Probably yes. In general, property lines are where they are, notwithstanding where people think they are. But the neighborly thing to do would be to talk to your neighbor first, to let him know your plans. This gives him time to adjust to the change (or to get a survey if he thinks you’re mistaken) before a fence
pops up on what he thought was his land.