Nickmanl I think you are forgetting that distance driven has nothing to do with the front door to plane door time. If you are on the east side of Tunbridge Wells the quickest route to Gatwick will be up the A21 along the M25 and back down the M23, a lot further than 20 miles and fraught with the potential travel problems on the M25/M23. On the west side of Tunbridge Wells you have to get through East Grinstead which can be a real nightmare.
I live not far from Tunbridge Wells, on the sussex side and I reckon the front door to plane door times of Gatwick and Manston are pretty similar for me as mentioned in my post further up.
However I also appreciate that any service from Manston is actually unlikely to go where I want to go as I generally don't do longhaul, although am contemplating trips to Southern Ireland and/or Valencia this year.
My own suspicion would be that a few destinations flown by a well known low cost carrier would be a start, and may suceed. For new carrier to start up from scratch they would need huge amounts of publicity and to put up with low loads whilst they built a passenger base. As EUJet proved it can be done, but it takes money and a lot of it to do it. And of course EUJet failed for whatever reason.