Is there a tap in the pipes that go up into the boiler? If so, turn it on gently to see if the white needle shows an increase in pressure. Once the needle shows around 1 1/2 on the dial, the boiler should be able to light. If there's no tap, I'd advise professeional help. The problem is, where has the pressure gone in the first place, since you have a symptom of a leak in the system?