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Our visitors have departed and I have been able to make progress. I had to remove the worktop to enable me to remove a sliding panel that slid down the front of the unit and the rear of the toilet pan. With this removed I was able to get to the cistern and the flush pipe. The cistern is secured to the back of the unit and is not on a metal support frame. The leak is from the joint on the base of the cistern, where the flush pipe enters the cistern. can't any form of a water tight gland/nut that attaches the flush pipe to the cistern, all I can see is a circular boss, part of the cistern, through which the pipe enters and the water flows out. I assume that inside this boss there must be some sort of seal that goes around the pipe to prevent a leak, but I can't see any. The leak is constant does not increase or decrease when the toilet is flushed
The cistern is fitted with a Geberit dual flush valve, the type which is also has an internal overflow to the toilet pan. Presumably to repair the leak I will have to drain the cistern and remove this valve to fit the seal (when I find what type it is), but looking into the cistern I can't see how the valve is fitted to the base and how the flush pipe is accommodated.