Boiler/thermostat help

[FetlocksBlowingInTheWind]

My boiler isn't heating up the house very well!
It was serviced last week and during the system test the radiators were burning hot, so no issues there.
Radiators all bled.
Today it's not getting above 14°C, even if I whack the thermostat up to 30 (as suggested by the BG engineer last week).

No BG appointments available till Jan.

Combi boiler, 4 bed terraced house, nothing fancy like underfloor heating.

Could it be the thermostat? It's just a crappy unsmart one, I'm not convinced it's talking to the boiler properly.

I'm rubbish at this kind of thing 😞

Should I look into Hive, is it worth it? Or just a new unsmart thermostat but then I do I need to find an electrician to install it?

Help me Mumsnet 🤷🏻‍♀️

Oh and BG man suggested a power flush which I disregarded because I've heard they're not really necessary and cost £1k. Not an option!

Boiler is about 9 years old.

Are your radiators hot today? If they are and the house isn't heating they may not be large enough.

Thanks for answering! They are warm but not hot. Possibly full of sludge?

I'm cracking on with replacing the thermostat so at least I'll know if it's that.