new build propertys generally have internal soil stacks to stop you seeing a unsightly thick pipe being stuck to the side or the back of your house, have any of your neighbours had the same problem??? if so it could be a problem with the drains i find dirgo valves not much use as heard of quiet...
generaly pressure problems in combi boiler signifies you have a leak somewhere, this leak could be that small that it has evaporated before hitting the floor showing no signs of a leak, if this is happening on a couple of radiators the pressure will go down fairly quickly. i'd go round checking...