Boundaries (where your land ends and your neighbour's begins) are NOT the same as fences.
If the neighbour owns the fence, they own the fence. They don't have to repair it, they do have to make it safe if it could cause an injury BUT they can do this by removing the fence entirely. Unless there is something in the deeds regarding a covenant,(and these are quite hard to enforce) there is no legal requirement to fence a garden.
If there is any issue with who owns the fence, then the fact that you have previously paid for repairs CAN indicate that you have assumed ownership. (personal experience).
Baseline is if you want a fence between your garden and your neighbour's, in general the only way to be sure you have one is to erect it yourself and maintain it yourself!