[Nick Beale]

I've been using Ultra since about 1990 (unlike you I'm only 300 km from London). Like you say, there's rather a lot of material...

ARC isn't the only abbreviation you'll find. Another is "Able Oboe" = AO = Air Officer in Command = Kommandierender General. "Nan Sugar" for Nachtschlachtgruppe, "Jig Dog" for Jagddivision and so on.



Signals weren't verbatim translations of the original, they are described as "reports to Allied commands". Partly this is for security - if the enemy suspects you have re-broadcast his exact message, it gives him a possible break into your own code. Some Ultras have lines from popular songs inserted into the text for added security.



Also, German code names were substituted by British ones. You need to find the message with the fist mention to find out what the German name was. "Einhorn" became "Salter" for instance.



Another reason was that the originals would be unintelligible to anyone but an expert. The originals are supposed to have been burned in 1946 (see the Channel 4 documentary "Station X" if you can get the video).

BUT in series HW1 are files with a daily selection of Ultras (maybe 4 or 5 a day) that were brought to Churchill's attention. These contain the verbatim translation and the signal as issued to Allied commands. You quickly find that there were codes within codes: GAMOZ and GEKOZ were Luftwaffe command echelons (I forget which) for instance.
Finally: I'd recommend "Enigma" by Sebastian Sebag-Montefiore. It's mostly about naval Ultra but it does explain a lot about how the system worked.