So I started running my bath today, nice warm water running. So I got into the running bath when suddenly the hot tap started running cold water. I switched it all off and tried again. Still cold. So I shouted help and explained the crisis. He went out, looked at the boiler, ran a tap came back and said "It works" So I said switch the boiler off, leave it for 10 seconds and switch it back on. He did and it started running hot again.
Dunno why that happened