Remember you should not be left in a worse position, insurance company will offer the glasses guide value and usually what it's indicated trade value would be.
You need to be able to replace the car with a comparable vehicle and so you will need the dealer forecourt value of the car from the insurer.
Trawl auto trader to find cars with similar mileage, spec and condition and use these as examples of the price you'd have to pay to replace the car. Don't just accept what they offer.
When out Punto HGT was written off I proved that it was impossible to buy another for the silly low offer they made and proved from other cars for sale at the time, that i would need a certain amount back in order to be able to replace the car with a like for like vehicle