Hi Simon and welcome
Your G80 was made by Millard Brothers (MILBRO) at their plant in Motherwell Scotland.
Production was from 1976 to 1982 after which the company went into liquidation.
The G80 had several different stocks fitted so this would narrow the production date of your gun even further.
The velocity when tested by Airgun world was 598fps in .177".
Spares are shown by Chambers in the Milbro section and John Knibbs has spares for these as well.
All the best Mick