Homebond aren't great on this kind of thing, their guarantee extends to the main structural elements of the house generally, foundations, walls, roof floors etc. Firstly you should write to the builder, outlining your problem and ask him to contact you within a certain timeframe. IF you get no joy from them turn to Homebond, they might agree to make a "without prejudice" inspection. In your correspondence be sure to emphasize "how dangerous" it may be and "potential for accident" it might speed up the response time. Failing that you'll have to resort to a solicitor I'm afraid. However if the underside of the stairs is accessible the solution could be straightforward.