Does it change when someone uses another tap?
If the shower goes cold the moment a kitchen tap runs or a WC refills, the problem is pressure balance, not the shower. On a system where the cold is on mains and the hot comes from a cylinder, or where pipework is undersized, drawing water elsewhere drops the pressure on one side of the valve and the mix swings.
A thermostatic valve is designed to protect against exactly this by shutting down if the cold supply fails, which is a safety feature rather than a fault. If your shower is not thermostatic, upgrading the valve is the single most effective change. If it is thermostatic and still swinging, the cartridge is the suspect.
Scale in the cartridge
Gloucestershire water is hard, and thermostatic cartridges are full of small passages and a temperature sensitive element that scale up over the years. The classic symptoms are a shower that takes longer to settle, a temperature that drifts during a shower, or a control that has to be turned much further than it used to for the same warmth.
Cartridges are replaceable and are usually a straightforward job once the supplies are isolated. Some can be descaled, though on a valve more than about eight years old, replacing the cartridge is generally better value than cleaning it and repeating the job next year.
Combi boilers and low flow
If the shower runs hot, then cold, then hot again in a slow cycle, and it only does it on the shower rather than at the bath tap, a combi boiler that cannot hold its minimum flow rate is a strong possibility. Water saving shower heads and partially blocked heads reduce flow to the point where the boiler keeps dropping out and firing again.
Descale or replace the shower head first, because it is cheap and it frequently solves it. If the cycling continues, the boiler itself needs looking at by a Gas Safe registered engineer. We are bathroom fitters and plumbers, not boiler engineers, so we will say plainly when the fault sits with the boiler rather than the bathroom.
Safe checks you can do today
Unscrew the shower head and soak it in a descaler or white vinegar overnight, then clear the nozzles. Check the filters in the shower valve inlets if it has them, as they clog with debris and starve one side of the mix. Make sure any isolation valves feeding the shower are fully open, since a half closed valve unbalances the mix perfectly well on its own.
Do not attempt to dismantle a thermostatic valve body while it is under pressure, and do not raise a temperature stop past its limit to compensate for a failing cartridge. That stop exists to prevent scalding.
Getting it sorted properly
Bring someone in if the shower has become genuinely unpredictable, because an unstable mix is a scalding risk, particularly with children or elderly users in the house. It is also worth a visit if you have already replaced the head and the cartridge and nothing has changed, since that usually means the pipework or the system type is the limiting factor.
Tell us your system type, combi, cylinder or electric shower, through the enquiry form and we can usually tell you the likely cause before we arrive.