Collett had a revised version of the earlier 5100 class procured from 1929. Since the class 5100 locomotives had the numbers 5111 to 5149 in addition to the number 5100, the first examples were given the numbers 5101 to 5110. The others were given numbers between 5150 and 5199 and between 4100 and 4179. There were a total of 140 examples were made, of which the last ten only left the factory in Swindon in 1949.

In the general dimensions, the locomotives corresponded to their predecessors, but they were a bit heavier and received various improvements in detail. The class 5101 locomotives were visually recognizable primarily by the external steam lines between the cylinders and the boiler and the rounded step between the running boards and the buffer beam. The larger coal bunker could be seen at the rear end of the locomotive, which the class 3150 also had.

Although the last order was not placed until after British Railways was formed, traffic on the branch lines on which the Class 5101 could operate soon diminished. As a result, fewer and fewer locomotives were needed for passenger service and some were used to push or pull trains on inclines. They were used, for example, on the steep lines in Devon or around the Severn Tunnel. After the last engines were retired in 1965, a total of ten were preserved for posterity. Five of them have so far been refurbished and made operational again.