Ulitimately the Class 20s lasted because they were ideal for the job. A pair of 20s on 42 HAA's ex Bentinck, Silverhill, Sutton, etc etc, had enough power, abilitiy to put it downa nd stop etc. A single 25 was underpowered, a pair over powered. Even a 47 and certainly a 56 and 58 were more rthan sufficient for "most" MGR working.
The 20s were solid, dependable and reliable........