I'd have been inclined to get in touch with the builder who constructed the property. (That's what I did when I had problems, and yes, I do accept that the NHBC itself is not the most helpful organisation.) However, buildings still under the original cover should have been constructed properly and I would have thought that any builder who valued his reputation would put this right. In terms of length of time, I had a bathroom installed by a reputable company whose plumber didn't tighten one of the (hidden) pipes properly. After six years, the leak had damaged some of the cabinetwork. The bathroom was not guaranteed for that long but I argued that if the pipework had been done correctly, the leak would not have occurred. The installer put it right. As memor maplin says, it's always worth asking - you never know what you'll get - but it's not necessarily an earful.
