How do I change the pressur eon my valliant boiler?

My Vaillant VCW T3W boiler stopped working this morning. The pilot light is on but will not fir up when put on constant and I get no hot water. This happened a couple of weeks ago and the gas man said it was just low pressure and all we had to do was turn a couple of taps till the needs on the gauge were back together again. I have done this but nothing works still - HELP please!!!!!!