I imagine that, in Law, it would be classed as a "nuisance", which might make it actionable............... but you really don't want to go down that route.
If it's what you think it is, it's "foul water."
It's against Environmental Health Regulations to discharge foul water in this way, even if it's only a leak.
Speak to the neighbours first as you say.
If that fails, ring Local District Council
Ask for Environmental Health.
They should take action if you're persistent.
(Play the 'Granny' card if necessary.)
Hope you can sort this :o)