Sorry I missed this fascinating thread. The Skyvan was indeed based on the Miles Aerovan (above) and it was said that Shorts paid royalties to Miles. The Van was similarly underpowered. We had an early version with Continental engines make a single-engine debut into Newtownards. The Astazou just didn't have the steam but quite a few of the final versions are still in service. Like most Shorts aircraft the Van was generously anointed with yellow anti-corrosion paste on faying joints, perhaps a legacy from Sunderlands. My 'apprentice' Alistair went on to become field service engineer for the Van and 330/360 family and said they gave little trouble despite their hard lives.
Most pilots loved it, Allan Deacon, later killed testing a Tucano over the North Channel, saying a Skyvan was the most fun one could have outside a double bed. The certification test flights were carried out on our 700 yd 09/27 runway at Newtownards, and traces of distance markings could still be seen last time I was out there. Landings were most spectacular into strong easterly winds, the loaded Skyvan descending almost vertically to thump onto the runway with puffs of blue smoke and stopping in a few lengths or so it seemed. Our five ton budgie did the job it was designed and built for and is still doing it very well, have a look at Youtube in Canada.