I agree with Shaun about the date. The Suntour rear mech should have a date code that might pin it down more accurately (FWIW). See here for a decoding of the code:
Maybe also the Sugino (see same page).
> 62 63, DC Davis Component
The 62 and 63 are the angles between the three sets of tubes at the BB, and Davis Components were a Birmingham (Sutton Coldfield)-based company making frame components used by a number of builders (and certainly in the 70s). Looks like the company was dissolved at the end of 2009.
PR 113 is most likely the Pat Rohan frame number. Not sure of the system: no obvious link to my Pat Rohan 531SL from late 70s.
BTW, I think the 451 is part of the address of Pat Rohan's shop: "451, Rayners lane, Pinner, Middlesex".