Every plumbing site names a most common problem and none of them cites anything. There is no UK survey ranking domestic plumbing faults by frequency, so this page uses the best available proxy and says so.
The proxy, and why it is reasonable
Cost guides are commercial documents. Publishers write pricing pages for the jobs people search for and book, and they do not write them for work nobody asks about. What gets priced in detail is therefore a decent indication of what gets called in.
On that measure the field is narrow: dripping taps, blocked sinks and toilets, running or slow-filling cisterns, radiators that will not heat, low pressure, and seized stopcocks. Dripping taps are priced by more publishers, in more variants, than anything else of comparable size.
Dripping taps
The most priced small job in the trade, and the one most often within reach of a competent household. A tap washer or O-ring costs £2 to £5 and having one fitted in London costs £60 to £95 including VAT, because you are buying an attended hour rather than a component.
Which repair you need is not obvious from the drip. A leak from the spout usually means the washer or cartridge; one from the base or handle usually means the valve or a seal further in. Traditional taps with separate handles have washers, while quarter-turn and single-lever mixers use ceramic cartridges and have no washer to change at all.
Blockages
The other high-frequency category, and the one where costs escalate fastest with depth. Checkatrade prices unblocking a sink or toilet with a drain snake at £60 to £90 excluding VAT, and unblocking a toilet at £75 to £150. Below ground it changes shape: rodding averages around £85, jetting around £185, and both together around £270.
Plunging is reasonable DIY. Repeated blockages in the same place are not a plunging problem, and neither is anything that backs up in more than one fitting at once, which usually means the problem is further down the run than a plunger can reach.
Cisterns, radiators and pressure
A toilet that runs continuously or fills slowly is a common call and a cheap one, sitting on the same labour floor as a tap. A radiator cold at the top is usually air and bleeding it is a household job; a radiator cold at the bottom is usually sludge and is not.
Low pressure across the whole house is worth reporting to your water company before booking anyone, because the cause is sometimes on their side of the stopcock rather than yours.
Seized stopcocks
Underrated, and the one on this list most likely to turn into something expensive. Which? warns that older brass stopcocks seize with limescale and that a seized spindle snaps cleanly when forced. Checkatrade prices replacing one at £140 to £240, on a basis the guide does not state as either including or excluding VAT.
The failure mode is what makes it worth attention: a stopcock is discovered to be seized at exactly the moment it is needed, which converts a scheduled repair into an emergency with the water still running.
Where the line sits
Reasonable DIY covers a washer on a traditional tap, plunging a sink or toilet, clearing hair from a shower trap, bleeding a radiator, topping up boiler pressure through the filling loop, and replacing a toilet seat.
Call someone for hidden leaks showing as wet patches on walls or ceilings, anything near electrics, soil pipes and below-ground drainage, mains supply pipework, stopcock replacement, persistent low pressure, and any job needing the water off for an extended period.
One line is not editorial judgement
Unvented hot water cylinder work is regulated under requirement G3 of the Building Regulations and needs a suitably qualified person, with compliance running through building control. Gas is restricted on a different and harder footing: under regulation 3 of the Gas Safety (Installation and Use) Regulations 1998, only a Gas Safe registered person may work on gas fittings and appliances, and doing it unregistered is a criminal offence.
That is the one boundary on this page that is not a matter of confidence or skill. It applies to a general plumber as much as to a homeowner, which is why asking whether somebody is registered is a reasonable question rather than an insult.
The cheapest habit
Whatever the fault, save it up. The travel, parking and call-out are paid once per visit, so the second and third small jobs cost close to labour alone. A slow cistern that is annoying in March and a dripping tap that starts in April are one booking, not two.
Common problems are cheap problems, right up to the point where they are attended twice.