Hello, my name is Jim and I am a qualified lawyer happy to help you today.
It would be a case of asking the contractor to prepare a report to support your case and then you should make a formal complaint to the insurer and if that does not resolve the matter ask them for a "deadlock letter", which is a letter giving their final response.
Once you have the deadlock letter the next step would be to escalate this to the Financial Ombudsman Service - they will investigate and liaise with the insurer which would hopefully result in a resolution of this claim. They can order the insurer to make a financial award for inconvenience if they have acted badly.
The service is covered by the Consumer Rights Act 2015 so you have a right to expect a reasonable service (from your insurer) which is carried out with care and skill - if you do not then the regulator will step in but it also means you can sue for any damages or losses incurred as a result of the insurer's conduct.
You can make the complaint here : https://www.financial-ombudsman.org.uk/consumers/how-to-complain
If the Ombudsman upholds your complaint then the insurer will likely agree to the shower door and panel cost. If not then you have the option of suing the insurer on the http://www.moneyclaim.gov.uk site and use the contractor's report to support your claim.
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