I believe that most of the civil certification was done by Transmeridian/British Cargo Airlines using Mashall's of Cambridge as the design authority. Both PS & PE flew with a Marshalls class 'B' registration during testing.
Thanks, in was able to springboard from that and get some more info. The aircraft received its provisional certification in March 1980 but without the stick-pushers were prohibited from carrying anything except cargo. The pushers seem to have been fitted in late 1981 having been tested on the third Heavylift airframe. Designed by Smiths and installed & tested by Marshalls. After full certification support personnel could be carried on the minstrels' gallery.
Two other certification requirements I missed earlier were a CVR and FDR. The reduction of the Smiths director to simplex reduced ILS capability to Cat II. A discretionary change Heavylift ( or its predecessors ) made was to remove three bunks to make space for spares storage.
Heavylift intended to display a Belfast at the Paris Salon in 1983; did that occur? Self-answer: yes.