Pressure problem on Glow-worm 30sxi?

Sorry Frank
No answer, just another pressure question relating to the 30SXI........
My central heating system doesn't retain pressure, no visible leaks on radiators or visible pipework etc
BUT
the overflow pipe from the boiler releases water slowly!

Code F21 within the Glow-worm booklet states...
PROBLEM....Lack of water fault (pressure is less than 0.5 bar)
POSSIBLE CAUSE....system leak or faulty pressure sensor

SO........my boiler works but I have to add water every day, is the problem a faulty pressure sensor which allows/forces water out of the system via the boiler overflow pipe or something else

Henry
March 2009