This may be off topic, but I hope to get some help here, so many knowledgeable contributors, grovel, grovel.
In my area none are on mains drainage, with no problems. However my next door neighbour has an obviously faulty septic tank. A few weeks ago it started leaking smelly effluent onto my property, I drew this to their attention so they said it needed emptying, and indeed did so.
I thought there was a problem, and so there is, less than a month later I again had the yuk coming into my garden. On speaking to them they replied it was only water overflow, I pointed out if that was the case there would be no smell.
They have dug a channel to divert the flow from my property so that it remains on theirs. Knowing "water" I reckon it will be back on mine shortly. Also it actually flows into a stream which carries onto the river Trent.
I do not wish to be a bad neighbour, but if it reoccurs what action can and should I take.
TIA