My present insurer is likely as yours and wanted me to put a price on for which they will then levy a premium; higher the value... higher the premium BUT in the event of a claim, they likely will argue if you place too high a value on it.
I'd not worry too much about it being from a good period or better than a new one etc, as 99% of Warwick basses have little if any difference in value due to their age. I've known early 80's Streamers go for less than 2000 era models and if you want a high value to tell insurance just look at the price of a new one! I know of at least one late 80's SSII that is for sale at £1100 and I saw one sell on ebay 2-3 month ago for £900. Personally I'd tell insurer £1000-£1500 as the top figure will secure you a used one easily and the bottom figure is what you can find them for if you look... alternatively, insure it for what you paid for it.