How can I fix pressure loss on my Worcester Bosch 28CDi?

Hello,
For some time now, there has been a constant loss of pressure. Quite by accident dripping was seen in the garden that proved to be coming from the central heating boiler. As a safety precaution the system was turned off and a gas engineer was called out. He replaced a valve, which, from what I can understand, functions when the pressure is 3bar.
The pressure is still lost even after having the suspect valve replaced.
The valve that has been replaced is made of brass/phos bronze and has 2 apertures. One of the apertures is threaded, while the other has an O ring. There is a red knurled knob marked 3 bar which, when turned opens and closes the valve.
When the system is turned on and the pressure is 1.5 bar it functions well, until the pressure gauge on the cabinet front reaches 3 bar. That's when the drip starts and the loss of pressure is seen. There is no drip or loss of pressure before then.
I thank you in anticipation of someone supplying an answer to this problem.
Jack