Before I take a hammer to it 
So had issue in December, heating person comes out, fits a shiny new thermostat, cleans diverter valve and says I may need a new one at some point. Fine, all sorted. Hot water is hot, happy thermostat
This week. And back to exactly the same problem 
Shiny new thermostat is set to come on for hour in the morning, two hours at night. Target temp in between is 5c (so basically not coming on)
2am, boiler kicks in so I check thermostat which is bleating "target temp 5c" but yet radiators boiling hot, and my hot water barely tepid
I want my hot water hot and my radiators cold (I like a cold house!!!)
But it's randomly kicking in constantly despite the thermostat forlornly saying "5c please" and when it's doing that the hot (ha!) water is resembling fridge temperature 
Ideas? Obviously I have rung the heating company but am awaiting someone to come out, and they say the thermostat is the only thing that triggers the boiler