I didn't realise you could have rads at one temperature and HW at another.

You clearly have your heating under total control.

Yes, if you have a combi boiler, you can set them separately.

https://www.boilercentral.com/boiler-advice/boiler-temperature.

I have had the radiator flow temperature set to 38 degrees for the last few days. It seems to be keeping the room at a suitably comfortable temperature. It's costing about £2 a day when left on for 24 hours. I have only had the radiators in my bedroom, the bathroom and the hallway on.

My central heating's come on.

Is summer officially over?

Mine hasn't.

And it never will.

It is a bit chilly this morning.

I had the heater on yesterday evening, for the first time in a while.

I've currently got the heating on 24 hours a day in every room, as an experiment. With my hallway thermostat set to 19 degrees, the radiator thermostats set to 23 degrees and the boiler flow temperature set to 35 degrees. I've had it set that way all through summer.

Because the coldest part of the house has never dropped below 19 degrees, through the summer, it's not activated at any point during the summer. But, the last few days, it's burst into life every few hours.