Removing Gala cistern lid: depress one flush button, hook a flat screw driver under the other button with other button still depressed, pull button upwards and it will slide out. Then you can easily slip out the other one. After that, it's a simple case of unscrewing the plastic screw sitting in the push button well. Sorted!
The Gala has an internal overflow so the water leaking into the bowl is your overflow water from the cistern. The cistern is doing this because the flush valve needs replacing.
You can adjust the speed of filling after flushing by lifting off the lid and turning the white screw on top of the flush mechanism, clockwise.
Getting the lid off is straightforward. Take out the two push buttons, they can be stiff with accumulated gunge but believe me they do come out (you might need a pair of grips or perhaps rubber gloves to get a grip on them). When the buttons are removed look down the hole and you will see a phillips type screw/bolt unscrew this and the lid (complete with the shiny button holding circle thingy) will lift off. What you do after that I have no idea 'cos I'm still trying to get mine to stop leaking into the bowl or taking ages to refill. Good luck!
Sounds like you need a replacement flush valve which you have to contact Gala directly for the information on getting one. B&Q - if that is where you bought it - don't stock replacement parts for it.
It's a job you can do yourself however......I have the same problem with my Gala cloakroom toilet. But I can't get into the cistern to have a look - the lid won't come off. Do you know if I have to shut the water off and remove the screws underneath the cistern just to get into the thing? The installation instructions for the thing are long gone.
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