David, Thanks for the tip. The levels are far too high and I was focusing on the lack of drainage from the Fosse as opposed to leakage from ground water into the Fosse.
Although we we have only half an concrete inspection lid I also made an assumption that the heavy rain was contributing. ie. Filling the Fosse directly.
I'll ask the chap that empties it today if he can advise.
Up until now we have only used it as a holiday home and I guess by the time we got here, mainly in the spring and Summer, (and lack of usage) this just sorted itself out once the rains had stopped. Now we have moved in we are seeing the real issue.
The loo seems to flush OK but the grey water (Shower, sink etc.) is backing up via the grease trap (which I have cleaned out). Cant find any blockage in the pipes though....
As I said we had a quote to replace and 10,000EUR seemed high for buring a plastic drum with a soak away - but talking to others this is about the going rate?