There are comprehensive circuit descriptions, including coverage of the operation of the video circuit, in the Nascom 1 hardware manual found on the same site as the circuit diagram here:-
http://www.nascomhomepage.com/pdf/N1HM.PDF
Among other things, the parts list in that manual usefully supplies the IC type for each IC circuit number, something the circuit diagram doesn't always bother to do.
For example, the main address decoder is formed by two halves of IC36, whatever sort of IC that is. The circuit doesn't actually say, but the parts list informs us that it is a 74LS139, (a dual 2 to 4 line decoder).