When the Western region began ordering diesel-hydraulic locomotives they had the German V200 design in mind and the Swindon-designed D800 Warship class bears a close resemblance to the German locomotives. Following a policy of dual-sourcing BR ordered a batch of these D800 class locomotives from North British in Glasgow. D833-D865 were built at NBL, fitted with MAN engines and Voith transmissions, in place of the Maybach-Mekydro combination of the Swindon built locomotives.
To separate the two types of D800 Warships for TOPS the Swindon Maybach engined units were given class 42, while the North British MAN engined locomotives were placed into class 43. Neither type survived long enough to receive TOPS numbering.