Simon,
Please allow me to assure you that a golden rule of an airline is to limit non revenue flying to as minimal as possible, non revenue in this instance referring to maintenance ferry flights.
Particularly during the winters there might only be a need for one chopper rotation to/from the island(s), it would take a chopper longer to ferry NQY/Heliport/NQY than it would to fly revenue Heliport to/from the island(s).
If you were to build your own home would you build the house in St. Erth but build the garage (to house your car) in Carloggas? Then when you get home of an evening, after a day's work, you can drive your car to Carloggas to then get a taxi home again to do the reverse situ the following morning and you can do this seven days per week, wouldn't it be great?
When I said "RAF vacated" i mean that it is no longer a MoD operated (funded) airfield, the military might be using it but they're only using it because it is there, the military won't have signed any long term contract with the airfield operator regarding continued usage thus there is no guarantee of continued revenue from military usage.
Continuing to operate St. Mawgan airfield will continue to be a drain on public funds, for years it was a turboprop airport with just the occasional jet visitor, indeed in crosswind conditions DHC7's would land across the 300ft wide runway! I still recall the NQY Airport fuel bowsers 'Sweaty Betty' and 'Fat Ann', I can't recall which of these they lit a bonfire under one morning to thaw the diesel in the fuel tank.
Meanwhile just down the road there is Perranporth, an already demonstrated viable aerodrome operation, ask any (honest) beancounter what the most viable option would be, to continue funding St. Mawgan airfield or sell the airfield off relocating NQY airport to Perranporth whilst investing in resurfacing/extending Perranporth's runway 05/23 to a maximum length, instalingl lighting and generally upgrading the aerodrome to an 'airport'.
But anyway, what your suggesting is ludicrous, PZE has a hangar and maintenance facility, it is being proposed that they relocate the heliport to St. Erth but you're suggesting that they forget to plan for a hangar in St. Erth and be forced to use an airfield on the other side of the county for hangarage!