The minimum experience for the sandpit was 4 to 5 years.If Nats bonded people,then their bond period would probably be over by the time Serco might be interested.
Nats can't retain people if they are not interested in staying,for whatever reason.Controllers are leaving from other stations to go to the sandpit,not just Hounslow aerodrome.Would they get any incentive not to leave.I doubt it.