It’s possible that a 1940s house never even had a fosse septique but rather a just large, usually concrete, hole in the ground called un puisard . A puisard is frequently some distance from the house with no ventilation pipe and often has a cover that is flush with the ground which looks like a large flagstone. The cover may be overgrown.
Many thanks for all your reply’s. My wife is back at the house this week and will go see the neighbours see if they remember as they are relatives of previous owners. Also see the Mayor and fingers crossed they might have the old plans as we have found out the previous owner was the Mayor for 26 years.
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I have an old house that was converted from a farm building in about 1912…I am now on the local towns sewage system and town gas, but in the garden there is a shallow square basin set built of brick stone & concrete at a low point in the half acre of meadow grass…this is where various metal channels meet to drain water away from the house…(not very visible, but definitely there) and they start beyond a cobbled courtyard where there are the remains still visible outlet leading from an old stone sink where they used to slaughter animals…probably before the building was converted.
I am still not totally convinced about what this was used for …Just a drain ? Or a reservoir to water the garden ?..If you poke around into the space using a stick, you can feel a large, dipped, disc, which rises up in the centre…which makes me wonder of at one time there was some sort of ornamental fountain there…particularly as the road nearby is called Chemin des Fontaines…any ideas anyone ?
Why not ask the neighbours… ???
We have a house nearby, named Les Trois Fontaines… because they have 3 wells/sources … 
which keep filling and the “overflow” has to be pumped away
My wife went to see the Major this afternoon unfortunately no plans of our house but to my wife’s amazement he got in his car and followed my wife to our house to help find it. Unfortunately they couldn’t but has arranged for a specialist to visit house tomorrow morning. So hopefully we can get it emptied and can save up to install new one.
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Hire a ground penetrating radar??
Fingers crossed the specialist might have one or at least a drain camera, not that it will be able to see a lot after 70 years build up😝