This clip was part of an Ellington 70th birthday tribute by Stan Tracey & the Big Brass put out by the BBC in 1969/70 - a Dennis Preston 'Lansdowne' production. There are also features on the programme for Ian Carr, Don Rendell, Acker Bilk and a few others. BBC4 have occasionally re-broadcast this programme over the past decade. UK Columbia also put out a 'Big Brass' LP which replicates the content of this show.
It's the only film of Harriott known to exist. He certainly recorded a whole series of shows around this time for HTV West/Harlech TV in Bristol with the 'Indo Jazz Fusions' but they are believed wiped. There's speculation that there is also film footage taken on the Continent (Germany?) but nothing has surfaced to my knowledge to date.