We have a small garden that is fenced in with neighbours either side. One side of the fence needs repairing, which we are quite happy to do, but I wonder who owns which side??
no, there isn't a standard rule, there is also the issue of who has historically taken responsibility for repairs. Your house papers might tell you more. The fence post rule is usually right but not always.
One quick way to see is, if you have a copy of your deeds with the boundaries drawn, the boundary line should have a small arrowhead along it. Whichever way the arrow is pointing, details who has ownership of that boundary/fence.