Maybe1day has I believe found one problem, that being the leaking gaskets in the bottom of the cistern. Changing these is a very simple job for anyone a little handy and cost minimal.

Once this is done it may or may not solve the other problem, but this will soon become evident.

A video from Youtube to get you started.

An easy and safe way to relieve some of the strain on a septic system is to divert the outflow from the washing machine into the garden. That water is, at worst, "grey water", and can be safely left to soak into the ground or into a drainage ditch.

Sorted the cistern with a pack of 3 from Brikomart, big rubber washers for 2,25Euros.
Bit of a pain to fit properly without removing the cistern, it requires slight chamfering of the inner hole. It has improved the situation a fair bit. Having a go with the plunger in the Loo also improved matters which points to a blockage somewhere in the pipe. The fact that previously it had blocked a couple of times after it had been stressed, which I won't go into, probably means some other technique is necessary.....