Boiler Pressure too High

[feilim]

I am having problems with boiler and radiators. The boiler pressure sat at zero for ages and the radiators would only come on if temperature was up full. This led to radiators too warm. The last time the gas engineer was out he tried to solve this and said to connect the filing loop back on and turn the gauge with pliers slightly. I done this and the pressure has went too high now. The heating still only works at a high temperature. Difference now is that the pressure is too high (at about 3) and I dont want it overflowing again and flooding my neighbour below. So basically how do I fix this to get my boiler working that I can choose the temperature of the radiators and the pressure comes down as well?