Great stuff, Michael. And so quickly, too!

Overnight, I think I've come up with the way of decoding these five-character abbreviations of Lat/Long. Your data from ADM239/26 is a codebreaker's dream, providing a 'crib' of a correct answer. I'll be back at TNA Kew later today trying it out on a full-size Admiralty Chart.

What we seem to have here, as well as a record of 255 Squadron's first ever attack mission, is an insight into Signals techniques at the time. If I'm right, these 5-character codes have the capability to define any position in Home Waters to an accuracy of about 0.08 nautical miles, which I make 162 yards. Brilliant - given that, at best, signalling in those days was via Wireless Telegraphy (not fitted to 255's DH6 aircraft), aldis lamp, semaphore or naval code flags. Or, in the case of 255 Squadron's efforts that day, fly back to base, report verbally and launch a second sortie!

Can anybody put a name to this 5-character system of nomenclature? If anybody else wants a go at the codebreaking: Concentrate on the last three characters and think "Russian Dolls" playing on a 5x5 chequerboard.