Have just realised you may be talking about the other pressure release, the one from the hot water cylinder. This is a common problem with mains pressure hotwater systems. which are not maintained regularly. Turn off the cold water mains supply to boiler - open the nearest hot tap and just let it run until it stops (leave it turned on). On the side of your cylinder is a brass hook looking thing, this is a pressure release valve which should drain into a black plastic funnel. Hinge the hook outward and hot water will gurgle down the funnel keep holding the hook out until gurgling stops and then let go (this may take several minutes. What's happening is that you are replenishing the air gap inside the cylinder which allows stored hot water to expand into - similar to the 4% expansion of water in the radiator system, only this time it's 4% of 185 litres (nearly 8 litres!) Now turn off the hot tap you opened earlier and then turn on the cold mains supply and your back in business. You may still need to replace the red 3bar pressure release valve to stop any drips, but I doubt it.
robbie repairs, December 2008