Hi IrenaP, that's a very insightful question. We initially had a good relationship with our builders and even recommended them to our friends (we had some frustrations during the project and there were delays but I thought this was part and parcel of a building project).
Problems began once we paid the final bill and moved into the house. A LOT of things went wrong within weeks of moving in - faulty pipeworks led to flooding in the kitchen and basement, the waterproof membrane in the basement walls was not installed properly and the water leaked through the walls and damaged our wooden floors, the bathroom shower flooded because the angle of the tiling meant that the water flowed away from the drain rather than towards it, our boiler failed on New Year's eve (we could've just been unlucky here but the builders were annoyed that I called them about it over the holidays...we have young children and need heat and hot water in the middle of winter. In the end, we were just desperate and called an emergency plumber), our man hole covers started rusting and staining the tiled flooring, the toilet pipework began leaking, spot lights in our bedroom would just fall out, our PIR lights stopped working... I mean the list goes on. We felt that a lot of the works were done to a substandard level and it took several phone calls, emails and texts just to get a response. And whilst they did eventually fix some of these problems, the builders said most of these were "maintenance" problems and not covered by our six year guarantee, and that we would have to pay them to get these fixed. It made me wonder whether they carelessly did a substandard job and just thought they could charge us more when it came time to fix them. And for some of these problems, we just paid local tradesmen to get them fixed either because of the urgency of the problem or because we couldn't face having to spend weeks/months chasing our builders.
It's helpful to hear from someone in the industry and thank you for asking your builder. Our builders can't get the light bulbs replaced without carving them out of the ceiling and recaulking (as I am told) and redecorating the ceiling either. I wrote them a letter of complaint and they have now responded, refusing to replace/repair the lights (because they see nothing wrong with the way they installed them) and have accused ME of bullying them because my letter said if we can't come to an agreement, we would have no recourse but to take legal action. You're right that these are just bathroom lights but I suppose they are the straw that breaks the camel's back!
If you have any advice from the point of view of someone in the industry, please do let me know! I'm not sure what other options I have now other than filing a claim.