OK, you prompted me to do some further investigation . . The web site for this is:

http://www.channel4.co.uk/

and since the damn place is all built on sub-frames I can't give a more specific URL into the programme. The way in I found was to go to the listings for LAST sunday, 3rd October, selected the programme (Untold: Britain's Slave Trade) and followed links from there. Ignoring the odd errored frame it threw up along the way. All looks interesting; clearly it's part of a "Black History" series, and its another programme in that series which is broadcast tonight ("Riot") rather than the second of the four slave trade ones.

G.