According to the folks at Bob Jackson Cycles my frame 12470 was made in 1977 (stock bike, not special order). One clue to dating seventies English bikes, of course, is the Reynolds decals (assuming the constructor wasn't using old stock up): they were different pre-1975, 1975-1977 (registered mark, but no TI), and 1977-1981 (TI Reynolds), when they changed yet again (C, SP, ST, Professional, etc.). You can look at this BF thread, this ClassicRendezvous page and this Gazelle Pages table for more information about that. Of course, that's no help for anyone whose bike lacks them.