Dale,
They need to build a new bl00dy airport!
To extend the runway, but only by a little, they either build into a housing development or build on stilts, cost effectiveness?
What is there worth saving at the Roborough airfield, the airfield (landing strip) is 476ft above sea level thus attracts the crappiest weather, I can recall a month of February during which PLH didn't see an aircraft for the entire month whilst cloud remained on the surface, I've watched 50 seater airliners almost tripping over the pylon cables, whilst getting below cloudbase, on Dartmoor whilst trying to find the airfield, even with a marginally extended runway the writing remains on the wall for the current airport!
If there is a genuine need for a Plymouth Airport then here's the opportunity for those in a position to do so to put their money where their mouth is, there is bag fulls of land available to the south east of Plymouth, much nearer to sea level than Roborough, ripe for development for a circa 6,000ft runway airport, can even use the Roborough runway for hardcore for a decent length runway, then the marketing people can try to encourage all sorts of operators, not just STOL aircraft operators, to their nice shiny new airport.
P.S. There currently remains a runway in a farmer's field mid (ish) point between Yealmpton and the A38!!!