The shower has gone weak. Not broken, just slower than it was, and you cannot remember exactly when it changed. That last detail is the one a plumber will want first, because low water pressure is a case to be solved by elimination, and the timing narrows the suspects faster than anything else.
Question one: everywhere, or one tap?
Go round the house with a jug and a watch. Run each cold tap in turn and see how long a litre takes. If every outlet is weak, the problem is upstream: the mains supply, the stopcock, or the pipe coming into the house. If only the hot side is weak, look at the cylinder or the boiler. If it is one tap or one shower, the fault lives in that fitting, and it is usually a blocked aerator, a scaled-up shower head, a failing cartridge or a service valve that someone half closed.
Write the results down. It sounds fussy, but a plumber who arrives to a note saying 'cold weak at all taps; hot fine; started after the water company dug up the road' will be at the answer in minutes.
Question two: is the stopcock fully open?
This is the most common finding and the most embarrassing one. Stopcocks get turned off for a repair and turned back on partway. Most homes also have an external stop valve at the boundary that the water company owns, and if that has been disturbed, everything inside the house suffers. Open your internal stopcock fully, then back a quarter turn so it does not seize. If pressure returns, the mystery is over. If not, the water company can check the outside valve and tell you whether supply pressure to your area has changed, which is a call worth making before you pay anyone.
Question three: what kind of system is it?
Older houses often have a cold water tank in the loft feeding the hot cylinder and sometimes the bathroom cold taps. On that arrangement, pressure at the bathroom depends on the height of the tank, not the mains, and a shower on the same floor as the tank will always be feeble. A ballcock that is sticking or a tank that has silted up will make it worse. A shower pump or a change to an unvented cylinder, which runs at mains pressure, are the usual fixes, and an unvented cylinder is a job for someone holding the specific qualification to install it.
Combi boilers and unvented cylinders run on mains pressure, so weak hot water on those systems points at scale inside the plate heat exchanger, a partly blocked filter, or in some areas simply a poor mains supply, which no boiler can improve.
Question four: what changed?
Sherlock Holmes had it easy compared to a plumber facing 'it has always been a bit rubbish'. Try to pin the change to something. A new neighbour with a big extension on a shared supply. A leak you had repaired and a valve left half closed. A water softener fitted. A shower pump that has become noisy and is on its way out. A very old lead or galvanised steel supply pipe that has been narrowing for decades from corrosion; those often need replacing from the boundary in, which is a proper job but transforms the pressure.
Handing over the case
By the time you have done the jug test, checked the stopcock and worked out what system you have, you can describe the problem in a way that lets a plumber quote sensibly rather than charging a diagnostic hour to discover what you already knew. There are 3,554 plumbers listed on this site; browse the ones near you, or set out your findings and ask for quotes. Include the jug timings. A plumber who reads them and phones with a theory is the one you want.
