For a history of the Short's Belfast see Molly O'Loughlin White's book: Belfast - the story of Short's Big Lifter. Published by Midland Counties Publications, ISBN 0 904597 52 0
The story goes, if my memory serves me correctly, that Shorts was developing the aircraft against an MOD order for 30 aircraft to serve the 'strategic' airlift requirements at the time (about '64). Shorts was designing the aircraft to meet both military and civil certification requirements.
The sterling crisis of '64 left the Labout gov't (who else) desperate for support from the IMF and part of the deal with the US was that they would help with support for the pound in exchange for purchase of the present fleet of ~75 C130K for the RAF. With that sort of commitment, the RAF obviously didn't need all that airlift and chopped the order to 10.
Shorts no longer had the numbers (i.e. money) to support the further development or the civil certification. Hence the engine was never developed to take the bigger prop (supposed to be 18' dia) and the aircraft has been underpowered throughout it's life.
Little known - the Belfast was the second a/c after the Trident to be equipped with cat 3 autoland. Same reason as BA specified it to land in the fog at LHR, the RAF thought it would be a good idea for a certain busy Wiltshire countryside landing strip.
lybid