No it was not the Accountant because it only flew a short test programme which was discontinued in 1958.
Perhaps HZ123 is referring to the Saunders ST27 which was a Canadian twin turboprop conversion (PT6A) of the DH Heron, and of which Aviation Traders had some part in the development. Photo on
www.airliners.net at Southend in 1971. Google Saunders ST27 Southend to view.
The Fire School seems to have started in the early 1960s in the location already described. In those days the aircraft in the compound included a Lincoln, Javelin, Meteor NF and a Tudor.