The insurance of irreplaceable heritage items does not, in my experience,
necessarily involve replacement value i.e. a notional market value is
meaningless if the same plates cannopt be bought again.

Instead, it is common for insurers to consider the conservation value, i.e.
the cost of conservation arising from damage to the negatives.  You would
certainly want this I think - it would not be very hepful to have a
notional insurance based on replacememnt value for loss only to find the
insurer saying that you still have the broken negs so he doesn't need to
replace them!

The most likely conservation treatment requirement arising from damage in
transit or copying is breakage.  Flood damage is a risk also of course.  As
a conservator not infrequently asked to treat damage plates, I can confirm
that treating and re-assembling a broken plate in a suitable form is, on
average, two hours work (it depends of course on the extent of damage) plus
materials.  For this I would charge 70 pounds for treatment and about 7.50
for materials, plus of course VAT.  This is a total of 91.7 pounds per
plate, giving you a combined figure of 917,000 pounds.  The insurer may
well be willing to put on a conservation price per plate, on the assumption
that only a few are likely to get broken.  More to the point, you have an
amount per plate avaiable to claim should only a few get broken (it
wouldn't be helpful to only get a payout if most or all were damaged).  The
insurers tend to like this because it is a calculation based on real costs
- i.e. the charges for conservation and materials, rather than foggy,
guesstimate market value

A favourite is for the institution/owner to put a notional market value on
the items for total loss, up to the premium that the institution can
afford.  This is done on the basis that, while the institution recognises
it cannot replace the items, it would value a fat chunk of cash from a
claim for loss and use the money for something they really do want to have
or do!  On this basis, you would definitely need a price from specialist
auction house.  There are such places in London in particular.

The bottom line is (as the insurer would agree) how much do you value this
collection?

I hope this is helfpul.
Chris Woods
Chairman, United Kingdom Institute for Conservation