The valuations provided for the Ploughman notes seem reasonable to me. Valuations for some of the rare types such as the Northern Knox £1 and Provincial Kennedy £10 might even be conservative.
A significant number of the more common Ploughman notes have appeared at auction over past 5 years and this has tended to depress prices. However, there is good value to be had now so it may be time to take advantage of the current prices
