Primary seals on double glazed units blown - insurance claim declined

mgdd

Six years ago, I had a local firm install double glazed windows and doors at the back of my property. The primary seals around ALL of the double glazed units are ALL now "failing" (black hot melt/bitumen leaking into the cavities of the units).
As the company are no longer trading, I am currently trying to claim against the insurance backed guarantee (valid for 10 years from installation). The insurance company sent an engineer to inspect the windows and to report on the nature of the defect and to provide an idea as to its cause.
After receiving the report, the insurance company now states "the windows are south facing and that the heat/sun has caused the bitumen seals around the double glazed units to melt". I am informed that under my guarantee "damage caused by sunlight" is not covered as stated in one of the exceptions clauses of my guarantee.
I don't understand how sun/heat damage, in the UK can be used as a reason for this. A neighbour two doors down from me used the same installers a year before me and he doesn't have this problem.
Any suggestions on how I can go about apealing this would be really appreciated. Thanks.