The short answer
Bring the old part if you can — matching it on the counter is the surest way to get the right replacement. If it's still fitted, bring clear photos of it and any labels, the make and model, and the key measurements: pipe size, thread size and, for tap cartridges, the spline count and length. For boiler spares, the boiler's seven-digit GC number identifies the exact model.
Bring the old part
Nothing beats putting the old part on the counter. Threads, splines, lengths and the shape of a seat are much easier to match side by side than from a description, and a broken part is still worth bringing.
Turn the water off at the isolating valve or stop tap before you take anything out, and put small parts — cartridges, washers, olives — in a bag so nothing goes missing. If the part has to stay in until you have the new one, take photos and measurements instead.
What to photograph
- The part itself from the front, side and underneath, with a tape measure or a coin next to it for scale.
- Any label, stamp or model number — on taps it's often on the body or under the handle; on cisterns, inside the lid.
- How it connects: the pipes going in and out, and the fitting on each.
- The appliance's data badge — on a boiler, cylinder or pump it gives the make, model and serial number.
What to measure, part by part
| Part | What to check |
|---|---|
| Pipe | The outside diameter: 15mm, 22mm or 28mm for most copper and plastic. Note the material — copper, plastic, MDPE or old lead. See 15mm vs 22mm pipe. |
| Threaded fittings | Measure across the outside of the thread; BSP sizes are named after the old pipe bore, not the thread. See BSP thread sizes. |
| Tap cartridges | Count the splines on the spindle, measure the overall length, and note quarter-turn (ceramic disc) or multi-turn. |
| Radiator valves | The pipe size (8, 10 or 15mm), straight or angled, and the make. Replacing a radiator? Measure between the centres of the two pipes. |
| Toilet fill valves | Bottom entry or side entry, and the height of the cistern. See fixing a running toilet. |
| Flush valves | Push button or lever, and the make of cistern. A photo of the inside of the cistern helps. |
| Wastes and traps | 32mm or 40mm, push-fit or solvent weld, and the trap's depth. See waste pipe sizes. |
| Central heating pumps | Make and model from the label, and the distance between the union faces — 130mm is the standard domestic size. |
| Immersion heaters | The element length — 11", 14" and 27" are the common sizes — and the 2 1/4" boss it screws into. |
| Expansion vessels | The capacity in litres and the pre-charge pressure, both on the label. |
A cheap pair of digital callipers takes the guesswork out of pipe and thread sizes.
Boiler spares: find the GC number
Boiler parts are specific to a model, and sometimes to a range of serial numbers, so "a Worcester combi" isn't enough to go on. The quickest way to the right part is the boiler's GC number — a seven-digit number, usually written like 47-123-45, on the data badge. The badge is usually on the casing, behind the front cover or under the boiler; the manual has it too.
Bring the GC number, the model name and the serial number, plus a photo of the old part's label if it has one. Gas-carrying parts — gas valves, burners, injectors — must be fitted by a Gas Safe registered engineer.
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Call ahead or order online
If you know what you need, order online for click & collect — it's usually ready in about an hour. If you're not sure, call the counter on 020 3802 3275 with the make, model and measurements, and we'll check we have the right part before you travel. We're open 7 days: Mon–Fri 7am–7pm · Sat 7am–6pm · Sun 10am–4pm.
If we don't stock it, we can very often order it in. And if the replacement turns out to be wrong, unused items in their original packaging can be returned within 30 days.
When to get someone in
- Gas: any work on gas pipes or gas appliances must be done by a Gas Safe registered engineer.
- Unvented hot water cylinders: safety valves and controls need a G3-qualified installer.
- Electrics: new circuits and most bathroom electrical work in England come under Part P of the Building Regulations.
For parts you buy from us, our supply & fit service can quote to fit taps, toilets, radiators, valves and controls in Greater London. For burst pipes and call-outs, our sister company Capital Plumbing Group covers London.
Questions
- Where do I find a boiler's GC number?
- On the boiler's data badge — usually on the casing, behind the front cover or underneath — and in the installation manual. It's seven digits, often written like 47-123-45.
- How do I know which tap cartridge I need?
- Count the splines on the spindle, measure the cartridge's length, and note whether the tap turns a quarter-turn or several turns. Better still, bring the old cartridge and we'll match it on the counter.
- Can I return a part if it doesn't fit?
- Yes — unused items in their original packaging can be returned within 30 days. Bringing the old part or good photos first is the best way to avoid a second trip.
- Is a 15mm fitting the same as 1/2 inch?
- Not quite. 15mm is the pipe's outside diameter; 1/2" BSP is a thread size named after old iron pipe. The two often go together — a 15mm tap connector has a 1/2" thread — but they aren't the same measurement.