At the risk of getting shot down here, I don't mind insurance companies trying to minimise the amount they shell out. It keeps my premiums low. But when you think you're being treated unfairly, it's no good saying it's unfair; you have to prove it is.
I had a similar issue when my motorbike was written off (and nearly me with it). A loss adjuster was appointed and offered a low sum. I sent a report just over an inch thick by recorded delivery, with similar models for sale across the country. With each, I showed features that were worse (or better) than mine with a realistic estimate of the effect this would have on price. I included copies of every bill and photographs of all the work (restoration) I'd done.
They paid me more than I bought it for. Evidence is what you need. Good luck.
