I tried to look in old post but can't find any information so I am
asking.

At this time of the year I am turning on the heating in my flat only
for couple of hour's in the bath room. All the radiators in the flat
are off.

The pressure on the boiler is low than normal about 0.7.
I know that when the hating is on it needs to be around 1.5 but isn't
the fact that only on radiator is on actually mean less hot water in
the system and there for less extract which mean lower pressure ? If I
top up the system and open more radiators the pressure goes very high
2.5.

Thank you for any information