Hi could you tell me why I have really hot water in my toilet cistern,thanks

Is the toilet tank the only fixture that's affected? When you have hot water in the toilet tank it indicates a cross connection between the hot and cold lines. This situation can occur in several ways. If you have had work on your pipes recently or the most likely culprit would be a one handled mixing faucet. To check this I would take and shut down each individual mixing faucet, starting with the bathroom. Have someone hold the flush lever down while you shut each faucet off so the toilet's running Shut the lavatory faucet down and check the water temperature coming into the tank. Still running hot? Turn the faucet back on and move on. When you shut off a faucet and the water starts to run cold to the tank you have located the source of your trouble. Pull and change out the cartridge, flush out the supplies and you're back in business. Good luck, Tonm