Water leaking from the cistern into the bowl it is usually caused by the ball valve being incorrectly set and is normally rectified by resetting

Or it could be that the 'float' has calcified and is sticking - either use a light oil to lubricate it or something stronger to get rid of the lime-scale build up.

The leak is between the 2 the rubber seals at the bottom of the syphon, the top one was covered in chalk and has cracks in it, I put some Silicon grease on it and it has improved temporarily until I find a permanent solution. Last night with the wáter turned off the cistern was empty this morning, after 6 hours...........

...or the float could have sprung a leak. Some are made in two sections and have been known to open up after a while.
Also the rubber washer at the end of the float arm occasionally needs replacing after years of service having lost it's flexibility and ability to make a tight seal when closed.

Considering the overall picture I'm inclined to the view that there probably is no major underground leak and water is probably being lost by one of the numerous faults previously mentioned.
A major underground leak would take very little time to rack up a bill of many thousands of euros rather than the sums mentioned.

As per my previous advice Get a local plumber. It is possible for you to cause irrepairable or very expensive damage.

"Very expensive damage" ...... It's a toilet!

The optimal result is buying a new seal and fitting it yourself; if you're taking it apart replace the valve mechanism while you're at it. Cost: about €40 tops. If you pay someone to do the same work, parts and labour, you're looking at €150-€200.

If you try the job yourself and destroy the toilet, you're facing maybe €120 for a new basic toilet and some labour to install it - so probably around €220 +/-.

So if you decide to not do it yourself you going to face a similar bill as if you tried to do it yourself and destroyed the toilet entirely. It seems to me like having a go at a DIY repair is a fairly safe, low cost bet.

My advice would be do not talk to the local council or any other official- very few self contained sewage plants (cess pits or septic tanks) work as the regulations require. If you start making noises to officials they feel obliged to do something offical - which will doubtless end up costing you both money and pain. Most authorities are happy to leave such things alone until someone rattles their cage, then they have to act according to the regulations.

Either locate the problem yourself or get a local self-employed plumber to help and do it all without any noise or fuss

That has consistently been my experience, and not just about cess/sewage systems. Round here (US/NC), the authorities turn a blind eye to pretty much everything that nominally needs a permit and inspection whether it is done by a contractor or the homeowner (this is not true for new-build). No questions were asked even when I demolished two chimneys entirely, which ran through the center of the house.

The licensed plumber, electrician, and heating/AC contractors I use have all asked me if I want permits to be obtained for the work - their preference, and mine, is consistently "no".

Ironically, the only exception I have experienced was when I consulted a septic system contractor about expanding the drainfield - when I asked how much bigger it should be to solve the poor drainage issue? he replied that he would do what the county permit said was required.

Complete toilets are available quite cheap from lee roy merlin if your one is beyond repair ,

the seats tend to be poor quality though on the cheaper ones.

Cheers Tony

It is not just the toilet. he has back up in the shower and the toilet. Local plumber is the only way.

OR maybe the toilet leaking water to waste is causing the pit to fill up , then causing the back up ?

Cheers Tony

Hmm could be

As I am not a professional plumber, perhaps you would care to explain to me how botching a DIY repair to the toilet might cause damage, expensive or otherwise, to the shower,

I would have posted some pics but it doesn't look like it is posible here. The cistern is emptying in about 2 hours, so over a day that's about 12 x 9 litres a consistent amount. I am keeping the supply to it disconnected today apart from when it is used.
As for the regs, here everything is in order and the Torremolinos wáter Co, Astosam provides wáter and sewage collection facilities. I called them once about 5 years ago about another wáter cistern under the garden. They said they would empty it if I insisted for 120 Euros but that it was for irrigation. They have a standard fee around 150 Euros now. They are supposed to be providing normal sewage facilities, this has been talked about ever since we moved in, The main is about 100 metres from us.

If its 100m down hill then it might be worth a connection - providing you don't need to cross too many other properties