"What Cornwall needs is a centrally located modest airport, perhaps a 7000ft runway but a longer one might be convenient for the occasional fuel stop before a heavy crosses the pond, a modest terminal, perhaps a freight facility, and with good road and perhaps rail connections and not a bl00dy great airfield & RAF camp the size of St. Mawgan, particularly one at an elevation of some 600ft.
So why not develop somewhere like the former RAF Portreath: "
A centrally located airport might be a good idea, but you could hardly call Portreath central. St Mawgan fits that bill far better. True, it might be a large airfield, but surely if you want to develop an airport, and one that includes features such as a freight facility, then space and hangarage is what you need. With the improvements to the A30 currently in progress, road links for St Mawgan will better those of Portreath. Agreed the local rail connection is paltry, but surely that is not a major consideration here?
Bear in mind that it is only the airfield that the MOD is disposing of, not the whole station. And the elevation is actually less than 400ft at its highest point.
I realise that you mentioned Portreath as an example only, but as an aside, it is also the site of Nancekuke, which as a former Chemical Defence Site responsible for the production of nerve agents, might not make it first choice for a regional airport!