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 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.

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.

Stocking a van for plumb2u.com toilet and cistern repairs means making a constant judgement call between carrying a handful of universal parts that fit almost anything, and carrying brand-specific parts for the jobs where nothing else will do. Getting that balance wrong either means a van weighed down with slow-moving stock or a plumber turning up without the one part a job actually needs.

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Noise is a real factor when deciding between the two. A side-entry valve introducing water above the surface tends to produce more splash and a higher-pitched fill sound, which shows up as a complaint on quieter properties such as flats with shared walls. Bottom-entry valves that fill below the surface are noticeably quieter, and swapping a noisy side-entry unit for a bottom-entry equivalent is a common upgrade even where nothing has technically failed.