WHBM - in 1970 I was station manager for Westward Airways and you may well be right about thye aircraft you saw, most substitute flights were supplied by Southend Air Taxis and sometimes on a push/pull cessna ( don't know its proper name!!) If we only had one or two passengers across the whole Heathrow rotation we sometimes used to tell the pax the aircraft was tech and put them in a taxi!! Frequently only carried two or three passengers, five or six on a good day there was simply no advertising. The fare was four pounds each way and five for a day return between Gatwick and Heathrow.
Those were the days.....