If it is a light bulb failing that would be down to the owner and classed as maintenance.
But the light fitting was faulty, you reported within the 2-year warranty (not a guarantee) and it was bodged by the foreman, presumably not a certified electrician, and has now failed completely.
So it should be the housebuilder's responsibility to fix it under the warranty.
Write to the regional office. Don't deal with site staff, especially those in sales who know nothing except how to sell new homes and fob you off!