My grandparents are having problems with their shower. The shower intermittently goes cold during use. It is a mixer bar on a combi system. It starts warm and then goes cold. If you adjust the temp on the bar it remains cold and seems unable to get warm again, regardless of where you position it. Hot water comes through to sink in the same bathroom. I am not sure of make or model of the shower, not much to go on I appreciate but any pointers would be greatly appreciated? My grandparents tell me the shower is hotter when central heating is off? Not sure if this is the case or even relevant? I was planning on replacing the mixer / thermostat wondering if it was broken or damaged??

All sorted, changed the cartridge controlling temperature.