Nice, looks like the later Mass Start, the Z prefix of this puts it late in the 60s... then again the whole Worksop Sun/Carlton numbering is one big head scratching exercise.
Early Massed Start from the Worksop era would be around 62, and have slightly different lugs.
Cheeky use of a much earlier Sun decal on the seat tube I see. Classy looking frame all the same.