Who's responsible for drainpipe repairs?

[MillyMollyMardy]

Dh has a flat that he stays in in the week. The next door neighbour's drainpipe is leaking and seems to be causing damp in his flat. The drainpipe runs down my husband's side of the fence but this seems to be because their fence has been put on one side of it (presumably so they could put the fence against the wall. )The drainpipe is currently pouring everywhere but into the drain.
I can't work out who is responsible; rainwater is from a roof that is owned by the freeholders (2 flats neighbour and their upstairs neighbour), pipe is on neighbour's house but dischargng onto DH's land.

I would expect it's the freeholders.

In a flat I previously lived in, my downstairs neighbours had a similar problem with water coming off the roof, they got the freeholder to sort it out.

However, the freeholders will recharge the amount they spend on fixing it to the leaseholders (in whatever proportion the leases say).

There are no leaseholders the flats are all owner occupied and the freehold is shared by the downstairs and upstairs flats.
What I'm asking is; the water is coming onto another person's property and causing damage who is responsible?