What is the problem with my Worcester Combi 350 boiler?

I have a Worcester 350 Combi boiler. During the last two to three weeks the pressure showing on my boiler pressure gauge has been climbing, if I drain out some of the water out of one of the radiators to reduce the pressure down to 1.5 bars, it will rise up again to 4 bars during the day ?? it does this twice or more every day.
Is the fault with a leaking filler tap ( there is a tap at the bottom that I have used in the past to fill up the radiators ) or is the problem elsewhere in the boiler, if so do you know what it is and the potential repair cost.