Hi both of you, actually I've answered but apparently it's not been displayed? ... Can you please double check... long story short my answer was:
The baseline is => You are the customer and therefore decide to go straight to the insurance or try to negociate with the builder first... Not a card I like to play but the "I'll call the insurer" card can be a pretty good incentive to fix the situation quickly as they can only be in one of these two situations:
A/ Not insured => They risk big money and they usually know it (illegal activity basically you can imagine the consequences for them...)
B/ Insured => The 10 years insurance costs big money so as many insurances imagine the increase when you declare a sinister... they clearly don't want that except if the damages are too expensive to fix (and consider that for that kind of insurance the no-claim attestation will follow them for up to 10 years so any claim will stick to your premium for a while ).