"INSURER will REQUIRE a written valuation."

Nonsense, unless it's a one-off or classic instrument which has a value enhanced by its rarity.

If it's a major manufacturer's 'normal' production model, a written valuation isn't necessary - the value of such an instrument is pretty well documented.

Jacqui, try Musicguard Musician's Insurance - I have two Martins, a Lowden and a Fylde mandolin insured with them, no valuations were required other than a declaration from me based on manufacturers' list prices, which I update each year on-line at renewal time. The policy also includes PL insurance.