bermudasun wrote:
I have just bought a new flat from and I am having problems with my
underfloor heating.

Put it in writing to the builder, get him to fix it. The problem will
be worse in 10 or 11 months when it is colder.

My problem is with the bedroom and hallway . The floor feels warm
however there the actual rooms are cold and never seem to get warm.

Inadequate heat output, able to maintain the steady-state heat losses
from the rooms at the design temperatures?
Floor coverings insulating the radiant floor?
Unoccupied, unheated flats adjacent?
New flat, damp plaster, screed & mortar, higher ventilation rates
required to dissipate the moisture?
etc., etc., ?????

If it's too cold now, it will be colder in very cold weather.

have tried running the heating over night to no avail. I have tried
setting the temperature on the thermostat in these rooms to maximum but
it never gets warmer than 20 degrees celcuis in the rooms. Can anyone
help and give me some advice??

20 degC can't be far off the design. Most UFH installed in screed needs
to be run continuously, the heat up time for the floor slab mass is too
long to operate intermittently like conventional radiator heating. The
air temperature can be lower with UFH, whilst still being comfortable,
due to the increased radiant output from the floor, i.e., the air is
cooler, so heat losses from you to the air are higher but heat losses
from you by radiation are lower, so you still feel comfortable.