You either have 2 problems
air in your system which is causing the presure to fall as it moves around the system, bleeding the radiators and filling by the filling loop to just over 1 bar should resolve but if there is a lot of air you may need to do this a couple of times.
or#
you have a leak ( not difficult in this weather) check all your pipe joints for leaks or even small weeps with absorbent cloth. Tighten if necessary and refill via filling loop as above.
Jo
December 2010
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