River Cottage

If you look on the packaging of all toilet related blocks & liquids they pretty much all say whether they are septic tank safe on the labelling. You'll be surprised how many ARE safe, I was.

No idea about what to do about the smells though sorry

have you got a cracked pipe or a loose seal on the back of the pans ?

maybe worth a look or even use a smoke bomb in the drain if you see smoke you got an air leak problem.

Is the tank  full?

If you have an access point on the pipework like a manhole ? I'd go with waz's smoke canister - thye you;ll SEE what's going on 

A loose seal with a slow drip which has rotted any wood/plaster behind the loo?  That's what my neighbour has !  Or maybe the pipe between your loos and outside are blocked?  We had this problem in that the builders had dropped rubbish down into the run-off which had blocked the pipe from the kitchen sink.  For years I poured gallons of Domestos-type stuff down because the back-up smell was from the cloackroom sink.  Eventually we sussed it! Thought we would have to have all the pipes up - kitchen sink pipes run under the floors to the outside - about 36 feet from one end to the outlet   I was ready to leave home!

The lack of a water trap will bring odours into the house. When the plumber was at work the place was really rank, and even now the vent over the loo needs a charcoal filter on it. So use the various methods prescribed and see what happens. If they don't work, call a plumber. My neighbour had this problem and it turned out their french drains had packed it in ...

Sorted?

There's a folk festival in Banbury soon ...........

Yes some feedback on the results is always helpful.

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