English Electric Type 1 (Class 20) Locomotives D8000 - D8127 (20001 - 20127)
Drivers Standard Preparation and Disposal Duties for Type 1 Diesel Electric
Locomotives E.E.Co 1000 H.P. Nos D8000 - Onwards
Issued April 1961 - BR33003/212.
More information can be found here
Wikipedia - Class
20
Class 20 Locomotive Society