New house taking ages to warm up

[CuriousAvocado]

Last weekend the temperature in the house dropped below 17C so I put the heating on to get the chill off (outside temperature was around 9C overnight). However it took almost 2 hours to get to 19C, and another hour to 20C! I don't think this is right for a new build so something must be wrong!

Some facts on the house:
-The house is a 4-bed detached with gas central heating.
-I've checked the radiator sizes and they're sized correctly for the rooms.
-The radiators are all hot to the touch with no cold spot.
-I've also checked the temperature at the flow & return pipes at the radiators (by touching with hands), and I can feel a distinct temperature difference so I think the balance is ok.
-The flow temp at the boiler is set to 70C
-The air permeability of the house is 4.7, which is within the requirement of 5

Everything seems to be fine on paper, which doesn't explain why the house takes so long to heat up!?
One thing I did notice is that during the heatwave this summer, the temp inside the house seems to be lower than the outside temp. I could feel a temp drop as soon as I enter the house, only managed to bring the inside temp up after opening all the windows and patio doors.

Can anyone suggest what is wrong with my house?

[New Home Expert]

Check the roof space for insulation.

My guess is it is you that is feeling the cold and need to put the room thermostat up to 20degC.

As your house was cooler in summer it does seem to indicate the walls and roof have insulation.
Turn the boiler thermostat up a notch too, as this controls the temperature of the water in the system.

[CuriousAvocado]

Check the roof space for insulation.

My guess is it is you that is feeling the cold and need to put the room thermostat up to 20degC.

As your house was cooler in summer it does seem to indicate the walls and roof have insulation.
Turn the boiler thermostat up a notch too, as this controls the temperature of the water in the system.

There is 400mm of insulation in the loft.

I thought a flow temp of 70degC should be more than enough for new build house, especially when it's not yet freezing cold outside.

It could be just me that feels the cold but I still don't understand why it would take so long to heat up.