I've managed to get some information from a contact of mine. Originally the plan was to film a documentary encompassing the making of all aspects of The Empire Strikes Back as was done with A New Hope. That is the documentary that is being discussed here. Then the decision was made to focus on the special effects of TESB and that became the documentary that was televised and released. It is unknown whether the original documentary was aired in its entirety or whether footage from it was used as EPK footage for TV shows of the time.
That's all the information I've garnered so far. I have made enquiries elsewhere so watch this space.