Leaking flush valve

Almost since I moved into my house I have had a slight leak from the flush valve on my toilet in my en-suite bathroom. It is not your normal loo with a cistern attached, but rather the sort you would normally expect to find in a public building, with a "button" on the wall connected to the water supply (in my case a 1500 litre roof-mounter water tank). You press the button and a measured amount (you can adjust it easily) of water flushes your toilet - the main advantage being that there is no wait for the cistern to fill, but you can keep flushing - ideal in, say, an airport toilet, or even an office toilet where there are multiple staff members using the toilet regularly.
The leak initially collected behind the button cover plate, and seeped into the wall, but I removed the plate and placed it in a drawer in my bedroom, and then the leak simply dripped down in front of the wall tiles, doing no damage.
As it is non-standard I have struggled to find a repair kit for this. The first I bought was a different brand, and the wrong size, but eventually I found the correct one, though it needed a specialised tool to remove the front plate - a 24mm box spanner. I could find a 25mm, but not the 24, so the leak remained for months, with an empty ice-cream container catching the dripping water. It was not major so I didn't worry too much about it, but whenever I went to the city I would try some hardware stores to see if they had the correct spanner, and even looked there during my trips to Peru.
I finally managed to get one in the UK, where I had not been for over 2 years, and took that to Peru before returning to my house in Brazil.

Repair kit

As can be seen the instructions for the repair kit (which, in the photo is sitting on top of a sheet of A4 paper) are minuscule, and consisted mainly of an exploded view of the system, while the kit comes ready assembled! It looked as though I had to turn the water from the 1500 litre tank off first, and since this was outside and only accessible using a ladder, I had put this off for a long time! However the leak appeared to be getting worse, and I decided I needed to get myself organised and effect the repair.

Since there are so many "how-to" videos on YouTube I decided to start there, so entered the details into their search box and up came a long list of solutions. The first one looked promising, but it was filmed using a mobile phone, which he set up on the sink about 2 metres from the valve location - and you could see nothing of what he was doing! He kept bringing pieces close to the phone, but they went out of focus, or he held them in such a way the camera didn't pick them up ... and this was the "top pick" most viewed solution!

The next two were no better - both were done by "plumbers" but they were working on a complete brand-new valve, on a worktop not installed in a working location, so basically were pretty useless. It is one thing to work on a "dry" valve, and another to work on something hooked up to the water supply - but both also claimed you had to turn the water supply off first!

I was about to give up watching and get straight to work when I noticed a clip by the manufacturer!!! Why it is not the number 1 clip I have no idea, but it was professionally done, covered several different scenarios, including if there was still a leak after you installed the kit and possible causes, and, best of all, showed that you didn't have to turn the water supply off first, but simply had to screw the valve in tight first which cut the water off!

So armed with this knowledge this morning I easily completed the work - well, removing the 24mm bolt took some effort as it was incredibly tight, and also when I removed the nipple and washer (top left in my photo) I found evidence of an earlier attempt at a repair, which appeared to consist of packed the area beneath the nipple with something like Blu-tack! Removing the complete old assembly (replacement one shown bottom right of the photo) was also not that easy, and took several attempts using levers and spanners to ease it out without damaging the old one. Put everything together, adjusted the water volume and ... no leak.

Final part of the job was to replace the cover plate and button, which were in my top drawer ... or not! They were in the second drawer, or rather the plate and screws, but not the button were! I searched high and low, including other parts of the house where the button had no reason to be - and then realisation dawned on me ... these had been moved by my rogue house-sitter in 2017, who had gone through all my stuff, and chances are that he had no idea what the plastic button was and may even have disposed of it, but whatever the reason I now have to try and get a new button to complete the repair. At least it doesn't leak any more!