I know someone who bought a new build detached house and was protected by an NHBC guarantee... or so they thought.
When subsidence started to occur they didn’t realise and they thought the cracks in the wall were the result of plaster settling, as can happen.
When the cracks got bigger, they called in the NHBC to look at things. Needless to say they didn't exactly jump to the rescue, and instead offered to merely observe the problem. When the cracks migrated from the ground floor to the first floor the house owners decided to get their own assessor in to take a look, because no one will know bull better than someone who may have been trained in writing it themselves!!!
In short if you have to claim don’t get upset, it gets you nowhere, just accept that it may speed things along hugely to get your own person in and start to make things happen. We go to a doctor to get healthy, we use electricians and plumbers for their expertise... don’t be your own assessor or claims handler. Always keep receipts and accept it's right to pay for your own assessor to work on your behalf. Yes it might cost but do not simply rely on your insurance company alone to come up with the goods. Many a time, they still need one massive great push to realise they are providing you with a service.