Your policy will quite clearly state whether it is an agreed or market value policy.
Also, check the value at renewal time - they have a habit of changing values while the rest of the policy looks similar.
It should also state what accessories or modifications are covered by the policy, and under what circumstances they will cover them.
You may need the PDS (policy disclosure statement) for the terms of when the accesories will be covered however.
Insurance policies are boring as hell to read, but it is very important to understand them, with the price of cover being as high as it is.