In the past standing in the garden near where we know the septic tank is located would be rewarded with a faint sound of trickling water after the loo had been flushed. I sort of noticed that this time this was not the case. So when we had the builder over for something else I asked him about the broken bit of concrete sticking out of the ground in that part of the garden. Some poking around with a ‘windy pick’ the next day revealed that the main pipe carrying all waste water and matter to the septic tank was totally blocked and that all those flutings and bath emptyings were going under the house but not exiting!
Long story short, we had to have a new septic tank dug. Six meters deep! Into solid lime stone at a cost of €5,000!!! Total savings wipe out.
I will put some photos in here at a later date.