C&M Britony 100 hot water goes hot to cold?

Hello,

My boiler has been getting progressively worse and worse and I'm literally getting just cold water out the hot tap now. the diverter valve seems to be working fine, and i tried putting some limescale remover in the secondary heat exchanger after cleaning out the central heating system and adding protector afterwards. There is a fault code that come up that means 'central heating thermistor flow faulty (open circuit)'

Someone please help, i just had fill my bath with kettle water, haha!

Andy