This is a very large and nice note. You have not mentioned which one you have, or perhaps you are referring to all 3 varieties.
I believe they are worth at least 180-200 pounds each in UNC condition, depending who you are buying it from, and this is only for those normal prefixes. First and last prefixes would command more. For the first series with Somerset's signature, I believe this would be worth more, imo.
I can see one with Kentfield's signature selling for 190 pounds, note is in aUNC condition.
Good luck to you.