Market testing another airport, or indeed expanding another airport in the area, seems to be an excellent way of wasting money, although I appreciate the owner of Lydd cannot see it that way.
Especially as I assume that whilst market testing the airport they need a second aircraft of each type sitting on the ground in case the one due to do the flight goes u/s.
There is a geberal problem with business plans in that you employ consultants to gather the data. They know what you want to prove. Aren't they going to do the best to prove it as otherwise you won't employ them in the future? Think how many National Lottery funded projects struggle once built. In all cases the market research said they would survive, but in reality the demand is not there.
Manston has not really been market tested properly, as it takes a long time to build up trade. Most people only fly once a year, so if you fly from Manston in year 1 you might go in year 2, but your friends might not go until year 3 etc.