definitely need to take a peek down the hatch… 
and the previous DIY could mean “anything-goes” … ah well… it obviously does go somewhere but … 
Could it be a cesspit which would have only an exit to suck the waste out??
Our house had no fosse, the waste just went into a well. The purchase price was reduced because of this.
Mine was the same, no drainage of any sort, all kitchen tap water went into a tank under the kitchen window so when it was full it would overflow down the drive and as for sewage, the previous owners must have put it down the well at the side of the house as well as there was no bathroom of any type We had a lot to do and then we were put on the mains drainage five years later which meant more work connecting up everywhere again
You can find people who will send a camera down to look…surprised your fosse people didn’t suggest that?
Me too! Thanks
Good luck.
Thanks 
When I purchased my house many years ago, the drainage situation wasn’t mentioned. Rather naively I assumed there was mains drainage, but it wasn’t until I was digging around in the garden that I unearthed a brick built chamber which is where all the waste water ended up - not a pleasant sight. During certain periods when I first owned the house there were odours, and just couldn’t figure out where they were coming from. So essentially what the original owners had installed was some kind of soakaway with everything just draining into the ground. As a result I installed a compliant system and, touch wood, haven’t had any drainage issues since.
One idea is to identify if you have any specific areas in the garden that remain quite ‘green’ all year round, as that’s what I had adjacent to the soakaway set up at my house. Because the area was getting continually watered with waste water it was thriving versus other areas.
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Oh how interesting! We’ve got an area like that - we will be investing next week- thanks so much x
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When we bought this house (as a maison secondaire) 30 years ago, the drainage system was very obvious - a single pipe from the kitchen sink going straight through the wall and directly onto the ground outside (my goodness was the wall damp!) plus a lovely little wooden shed outside sited on a mound. This latter had the traditional yellow pages on a string behind the door plus a plank with a hole in placed over a deep hole in the ground. It was surrounded by the healthiest most prolific shrubs I’ve ever seen 
The reason I’m mentioning this is because when we got the fosse installed, the land drains went down the slope of the garden, so the bottom of the garden was always green and verdant but the fosse was completely within norms, If you have a patch like that it could be just like @letsmile 's situation or it could be the end of the land drains. Is your garden on a slope, and is there a lot of it?