Several people seem to confuse "knowing the legal landscape" and "taking legal action". I wouldn't remotely suggest legal action is appropriate at this stage, but I don't think you can begin to negotiate a commercial outcome without knowing how legal action would be likely to play out, because the large builder is almost certainly going to know that and will negotiate with that in mind.
If you know that you have a strong case that is likely to succeed, even if you never want to take legal action, you can be much more aggressive in negotiating. If you don't have a legal leg to stand on, then you're likely to just be perceived as a joke if you start making demands.