Re comment by ExSimGuy, a totally unrelated motorcycle accident to the south of the airfield closed the A259 and prevented exit (for low vehicles) via the very shallow railway bridge mentioned. This effectively led to the crowd being locked into the field for the best part of 5 hours, until a route was opened across the runway and out along a narrow track to join the A27 well to the west.
Even though the event provided loos, plenty of food and alternative entertainment to occupy the crowd during the wait, I wish to pay tribute to the patience and understanding shown by all present. It was a difficult and traumatic time (especially for those with children) and I saw nothing but calm and tolerance under the circumstances.