Yes the rads are hot so boiler would seem ok. I only have one
thermostat and that is in the hall. When it is mild, this clicks off
when 20c/21c is reached. When it's cold it never clicks off as it never
gets up to temperature. On the old boiler, I remember when elderly
aunts used to be expected, we used to click it up to 23c and it reached
that temperature. Mind you, it was very warm!

I've tested thermometer readings in each room - most are 18c, one is
20c. So it looks like draft proofing, but I come back to my original
question, how can I get this tested? I'm reluctant to have cavity wall
insulation if there's a chance that I end up with condensation when
there might be little heat escaping anyway?

Thanks