It was the late Colin Cooke who told me how rare they are. In the mid-90s - when Spink didn't list the two varieties and had a general price of £6 for 1940 - he advised me that my Unc example would retail for at least £20...
... so £25 for an Unc example is a damn good price!
I'm also running out of space - I might have a closer look at a cabinet - for now I've moved half my collection to a secure facility elsewhere (this also spreads the risk - I'd rather have a 1% chance of losing half the collection due to fire/burglary etc than a 0.5% chance of losing all of it).