Thanks for the input chaps, much appreciated and very intetesting. There seems to be a few Suns on the forum recently.
I checked the rear derailleur on Velobase.com. It's a "Huret Allvit 2nd generation, early '70s > early '80s". IF it's original to bike.
Another big "if".. IF Worksop built Sun frames used the same dating system as Worksop built Carlton frames, from 1968-72. (In the link it makes reference to 7 digit numbers, after date letters and before "W" codes). www.carltoncycles.me.uk/details/dating.htm
An it's not the one in the 1970 catologue.
Could it be remotely possible that it's a 1971-72 model?..
*2008 Rock Lobster Team Tig SL, MTB.
*1956 Raleigh Superbe 3 Speed
*Mid '80s Dave Hinde SL Lo-Pro/TT.
*Mid '90s Menet SPRL "team bike" Neuron 7410 DA.
And a few project bikes.