The original license-built MAN engines of the Class 21s had proved unreliable , so in 1963 locomotive D6123 was sent to Paxman 's Colchester works to receive a new Paxman Ventura V12 engine . The machines were produced from 1963 onwards from the North British Class 21 by replacing the original unreliable licence-built MAN of the Class 21s with Paxman Ventura V12 engine at Paxman 's Colchester works .