Rather than an airborne APU (which the Lancaster didn't have
AFAIK), it's likely to be the engine that drove a generator for a ground power unit.
These were mostly derived from motorcycle engines, and "the E C Co Ltd" is almost certainly the Enfield Cycle Company who (according to Wikipedia)
"produced 250 cc and 350 cc flat twin 2-stroke petrol stationary engines during the war which were used for generators and other military uses".
https://en.wikipedia.org/wiki/Flat_twin_engine