Well progress has been slow but thanks to some helpful and knowledgeable people (here), progress has been made.
So what do I know now that I didn’t know before? Well, I burned a ROM that just tried to access an imaginary I/O port. When I ran that and watched the _IOREQ_ line, I saw it going up and down which suggests that the program is running correctly from ROM. I put one of my screen-filling ROMS in that just access ROM and screen RAM and didn’t see any _IOREC_ activity so I think I can safely say that the processor is up and running.
About a screen.
I wanted to see if the screen output I’m seeing is coming from video RAM or is being created on the fly by the hardware fault.
So I pulled out a video RAM chip. When I powered on the machine and reset it, the pattern on the screen was different to what I had seen previously. I took another RAM chip out as well and saw a different pattern. Each RAM chip on the Nascom-1 is 1 bit wide and so unfilled sockets will make the corresponding bit float high when accessed.
I think this means that the video RAM is being read correctly or at least feasibly by the video circuitry.
So now I’m looking at the memory addressing and swapping chips to see if the symptoms change. I’m also waiting for some spare 74LS chips to arrive so I have some contingency.