Out of the several hundred people each day who travel by train from Darlington to London, I wonder what proportion then trogs across town by Tube to take another train from London Bridge/Waterloo/Victoria/Paddington etc. to places like Kent, Sussex, Essex, Hampshire and the West Country. Probably not enough to fill planes to the likes of Manston, Lydd, Gatwick, Heathrow, Southampton, Bournemouth or Bristol. However, if you add in those catching a flight overseas, then maybe. Heathrow is a non-starter, obviously. But surely Gatwick still has potential. bmi baby gave it a go, but one flight a day with a 737 wasn't right. The obvious candidate to me would be flybe with Q400s. Maybe flights to both Gatwick and Southampton.
Right, that's the pie-in-the-sky pensive bit over with. Now for the reality. What about people in London and the South wishing to travel to County Durham or the Tees Valley for business or leisure? Yes, there are some! I used to work in London and often travelled to towns and cities around England. I far prefer flying to the train, so always took the plane when I could. Huddersfield or Liverpool? (= fly to MAN then direct train). Durham? (= fly to NCL, then Metro and 10 minutes on the train). It is easy to connect to public transport, which is more important for inbound pax (most returning locals will be picked up by rellies, but inbound visitors will rely more on public transport). With one bus an hour from MME to Darlington, it barely rates as an option. Therefore, inbound visitors will instead fly to NCL or take the train all the way.
The free bus that One North East funded went empty most of the time, because there were no flights coming in most of the time. Then it was packed solid when one flight did come in. If they had been foresighted enough and flexible enough to provide a dedicated large enough bus just to meet each flight, then provision would have more closely matched demand. If they can do it at "Stockholm"-Skavsta, why not at MME?