why am I losing pressure on my Main Combi 24 HE?

Our boiler is barely 18 months old but we've had constant problems with the noise and pressure. it is now at the point that I'm filling it up twice a day. We've had the manufacturer out twice, the first time the engineer noticed we had a small leak on one of the radiator valves. Once replaced that solved the problem temporarily. The second time, a week later, the engineer fitted a new pressure valve/balloon(??). still no joy. we then used a local tradesman who advised us the noise was due to the pump needing to be replaced, that solved the noise problem but not the pressure. Now I have to fill the boiler up to standard pressure first thing in the morning for a wash and even when i get home for barely a sink full of washing up water, a bath is out of the question. All our hot water is draining outside into the window box below. All we're getting is steaming hot flowers outside our dining room window.