In his introduction to the book "Dad's Army The Lost Episodes", published by Virgin in 1998, David Croft explains the policy behind the wiping of tapes. Firstly they were expensive and therefore a department was set up at the BBC to service and repair tapes for re-use. Secondly an agreement with Equity did not allow the repeating of programmes more than two years after their first transmission (in order to keep actors in employment by working on new shows rather than allowing air time to be crowded with repeats). Therefore there was no point in keeping tapes longer than two years.
He then goes on to say: "One of the factors that helped save my programmes from destruction was the fact that I was on the staff and therefore always present when a piece of paper came round requiring a producer's agreement for a show to be wiped. I always contrived to withhold consent. I can only conclude that when the necessary form appeared for these programmes, I must have been on holiday".
The series was recorded on 16mm film for transmission in NZ & Australia. Over the years, he says, he has sent letters & telegrams and even paid personal visits to search for copies but none turned up.