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Surly 2 seconds with an m10 nut, which all bikes shops surely have, and they could have solved it. Looking at the CRC website, it states the keys have a unique serial number which implies, to me, that it's not a universal key.
Cranky-
From a quick google it seems to be a universal key, if you purchase a set the key would work on any other set. The unique serial number just means that you are a "legit" customer who purchased their product so you can buy a spare key.
The staff at evans just did a lot of "hmm" and "arring". The mechanic there ended up measuring the inside of the nut with callipers and said "its around 10mm so M10" but 3/8 is very close to 10mm. I ended up just threading my nut onto a friends bike with a known axle nut. I thought it would take a few minutes max for a bicycle shop to tell me conclusively what axle nuts I had but not at evans.
kentikins
(Although a secondary locking solution)
I Just came across and purchased some Delta Knoxnuts in the clearance section of CRC.
They're similar to pinhead axle locks however they are shittier in terms of they have a universal key but they do have a spinning collar just to add annoyance to theives and they're very cheap at the moment at £7.99 for a set (single wheel).
On another note I did spend half an hour in Evans to see if they could identify my axle nuts as m10 or 3/8 but ended up with no answer as one worker said maybe 3/8 and his colleague saying maybe m10.