Hoping someone can help! Paging @PigletJohn 
Neighbours extended the back of their house (kitchen) within planning regs 3yrs ago. The builders installed a soak away - a requirement.
I've just read it ought to have been 2.5m from my boundary and I'm fairly certain it isn't. We're both 2-bed midterraces and our gardens aren't very wide 
It was definitely closer to my fence but is now covered by lawn/ possibly decking so I don't know its exact position.
The problem is my well-installed fenceposts/gravelboards are rotting in the region of it. Could it be coincidence? The rest of the fence including the considerably older fence around the rest of the boundary is unaffected and standing strong! The decking they installed (covering the concrete right of access path ) between the house and up to/over the soak away) is also attached to my fence without permission - not sure if that's a factor.
I have a very strained relationship with them due to noise issues (they are very noisy and it's going through another cycle) so wanted to get my facts straight.
Would Planning have a record of the position of their soak away? Or do I just need to accept it and replace my fence posts?
Thank you!