did you scan this thread?:
http://www.bikeforums.net/showthread...jackson+serial
there might be something in there, including some iffy guesswork by yours truly. Too bad BJ doesn't seem to be offering the build cards for 25 pounds anymore (or so the rumors go), still if you ask nicely they might respond.
My guess it that it's a late 70s/early '80s frame, but the 'conservative' touches like the top-of-BB-cable guide can be misleading, I think BJ used those well past when others had moved on.