Simplistically, the band need to list all the instruments by type, then visit a web site or store and see what it would cost to replace them "new", and this would be the valuation - no expert needed.
Similarly, if you had to replace your library, visit a good web site selling music and see how much it would cost new, and this is the valuation.
However, insurance companies exist to take as much from you in "premiums" as they think you can afford, and pay out as little as possible should you dare to make a claim. The difference is called "profit", and their shareholders love "profit"!
Another approach to the instrument valuations could be their second hand value, i.e. what you would pay on a "like for like" basis (10year old Sovereign cornet replaced with 10 year old Sovereign etc); for this I would suggest looking on E Bay and noting the final price at auction as a guide.
On the other hand you will not be storing all the instruments in one place (players tend to take them home to rehearse?), so may be their repair value is all you should be worried about.
But surely the biggest insurance concern is the horrendous amount of Public Liability Insurance demanded by some local authorities before you can play on their land? Currently the asking figure seems to be £3Million, has anyone else had greater amounts demanded?
Alan Lafferty
Band Cecretary, St Gennys Silver Band
Organiser, Bude Metric BRass