Diminished Value is not a term that insurance companies like to hear unless it is in thier favor.
Example: I had a rebuilt title 98 Buick Riviera that I used as a daily commuter. I bought it at an insurance auction, it had a damaged front bumper cover, left headlight and fender. I repaired it, less than 2K for parts, (this was a few years back). I drove it for years and then gave it to my son who was promply rear ended by someone talking on their phone instead of paying attention to what they were doing. They had no insurance, ours covered it on the uninsured motorist coverage. At the time it was worth about 4500 per the blue book, they paid 2k! The reason?? it had been totaled before! Did they give me a 50% discount on the coverage costs all the time I had it insured with them? NO!!!
Rant over.... Get a lawyer. That is the only way to fight it. IT will not be worth it. The legal fees are going to more than surpass any gains you will get on the diminished value claim.