Ok I also have no law expertise but, opinions I have aplenty.
From your posts it sounds as if you are sure the foul water is being dumped on the land somehow. I think you need to confirm this is the case first, since if sewer water or indeed anything else is taken away then the bill is legitimate.
If there really is no foul water removal by the company then as mentioned in posts above there would need to be a septic tank somewhere which you periodically have emptied.
For sure an ombudsman is a good start, but beware, I was "using" one once and discovered they don't really take your side against the powerful commercial concern. They say they are there to mediate and bring the parties together but I found nothing changed for ages, followed by a letter from him saying as he hadn't heard anything he took it all was ok and settled now !!! But it's better than nothing I suppose.