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Fixing centres, meaning the distance between the two mounting holes in the pan, is the measurement that actually determines whether a seat will fit at all, and it varies enough between manufacturers that assuming a standard spacing is a common source of returned parts. The two most frequently seen spacings in the UK are around 140mm to 145mm and 160mm to 165mm centre to centre, and a seat bought for the wrong spacing simply will not sit over both holes.

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The flush valve tells a different story. If water is draining slowly from the cistern into the pan even when nobody has touched the button or handle, the flush valve seal or siphon washer has likely perished and is no longer sealing the outlet at the base of the tank. This shows up as a cistern that refills every few minutes with no obvious trigger, sometimes described by customers as topping itself up rather than running continuously. On older gravity-fed systems built around a siphon rather than a valve, a split diaphragm inside the siphon body produces the same symptom.

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The practical test is simple: add a few drops of food colouring to the cistern water and wait without flushing. Colour appearing in the pan points to the flush valve or plumb2u.com siphon. A water level that keeps rising past where it should sit points to the fill valve. Checking both takes two minutes and saves a second visit.