I attach some more pics of the frame & badge. Not sure if it's easy to see from the pics, but the fork-shoulders are distinctively sloped. Does this give a clue about age/model?
Also, the brake callipers are Weinmann 500's and the casting stamp date them being made in 1979. Can't be sure if the brakes were on the original bike though!
The frame appears to have been re-sprayed. I say this because the Falcon badge is glued onto the frame and not rivetted. I got this bike off a chap I worked with in the late 1980's. He had owned it for a while and I picked it up for a song (£15). This is what makes me think the frame must be much older than the '80's. My gut feeling, combined with advice from other forums, suggests it is a 1970's frame, but i'd really like someone to nail the date from the engineering.
I haven't found a frame number, but it is possible it is under the re-sprayed paint.