Hi there,
The diesel hydraulics were designed with major component changes in mind. Engines, transmissions and bogies were changed as necessary because of failures at the WR depots at Old Oak Common, Bristol Bath Road, Plymouth Laira, Cardiff Canton. They would also be changed when they got to their overhaul hours target. In the case of the Warships, the engines and transmissions had to be the types they were built with, but the bogies were completely interchangeable between the two manufacturer's classes. The locos only needed to go to Swindon for scheduled body overhauls and major work beyond the capabilities or time availability at depots. So when a loco arrived at Swindon for overhaul, the engines, transmissions and bogies could be sent out as depot spares if there was still significant time to their next overhaul. With the amount of component changing that went on it was quite rare for a loco to arrive at Swindon with all major elements due overhaul. It was "an interesting time!".
Bazza