While I cannot comment too much on your claim I am curious to where you get the information that Aviva are the company with the most claims to the ombudsman?
I do not believe this is correct in any way shape or form and can provide verified statistics as proof. While I believe that they may have a good number of claims this would be largely because they are one of the largest insurance providers and therefore pro-rata it stands to reason that they will accumalate claims more than smaller companies. If fact due to the professionalism and size and experience of the company they will often receive fewer complains that some oif the smaller companies
What you will also find that is normally on over 30% of complaints that go to the ombudsman it is due to non disclosure or as you put it "false declaration" which generally comes down to the ombudsman siding with the provider.
That said, it does sound like you should take this further for resolution
My best advice I can give to you is to follow the official complaints procedure and include all communications and documents you have at your disposal in your letter of complaint to back your case. The provider will have 8 weeks to reply to either refuse your complaint (Final Response) or to resolve the issue. Once they have given a final resopnse then you can approach the ombudsman. If you do not follow this procedure then the ombudsman will most likely place the complaint for you meaning that you are just delayed in the process.
It does sounds like you have had a bad time with the provider and they must surely have given a reason why they have discounted the weather report and why the surveyors report was actually a "mistake" They must have held some further evidence that the report was mistaken that you have not seen....they should be able to provide this for you....or not.
Follow the correct procedures and worst case scenario the ombudsman will get the case look at it in detail for you