I found my radio code in the front page of the 'Service Book'.
However I found the procedure for entering it very confusing because as I entered each number the screen spelled out C-O-D-E rather than displaying the number I pressed.
Well, hah, funny story - the first thing I did, when we found we needed the code, was to check the manual and books, and I didn't find the code anywhere in there - hence all this searching.
Turns out I'm an idiot, and it WAS in one of the books somewhere, as my partner found when she had a look herself.
She's being very nice about it, which frankly only makes it worse.
Anyway. The only conclusion I came to was that most of the online methods of finding a Jazz radio code are extremely variable, and probably not reliable at all.