I was at the 1959 Halton B of B air display
I was there, too, Zetec, and was No 2 in the Halton Chipmunk formation team (do you also remember seeing 2 loops in close Vic!). Your memory of the Devon's aerobatics is correct; it was an ETPS aircraft and there were 2 pilots, one was certainly Lew Cockerill (sp?); I remember the other as being a foreign ETPS student whom I thought was from the Italian Air Force, are you sure it was Zurakowski?
The reason for the second loop was that the aircraft scheduled for the "guess the height & speed competition" dropped out and the Devon was asked to fill in, having been told to fire a Very cartridge to signal the "guess point". They did this at the top of the loop and simultaneously feathered a prop, so that it looked as though it had been a single-engine manoeuvre.
The aircraft had originally been scheduled only for the ground static display but when it arrived they were asked to join the flying programme as there had been a few cancellations. They don't seem to organize displays that way any more. 