First of all your home insurance is for storm damage etc and not maintenance issues such as this.
Clearly, the fibreglass roof is not "fit for purpose" having cracked this quickly within 4 years.
Your warranty, as you have discovered, is pretty much useless and the self-builder wasted his £1,000 in my opinion. I expect if you dig down the policy you will find all manner of get outs and exclusions excesses etc.
But..... it appears clear that water is penetrating the structure (a breach of the "waterproof envelope" - the waterproof part being the fibreglass roof) and in all likelihood it will continue to do so, degrading whatever it comes into contact with. Ultimately, this would make Build-Zone's costs higher than they might otherwise be now if they fit a new roof.
I would write one last time to Build-Zone and tell them you want a new roof fitted as required and also clearly covered by the warranty or a letter from them saying you are in deadlock so you can complain to the Financial Ombudsman Service (FOS) which you should then do.
I would be amazed if the FOS didn't rule in your favour.