Totally dead, no screen display (blank white screen). For this I used the Fluke Z80-S2650 adapter to check out the ram and rom, and located two errors. The rom mapping was incorrect, with a large range of locations returning random values. Examining the board (since I have no schematics), all of the chip select lines meet up at IC41, a 74LS156. Replacing this put the memory map back to normal (it seems to map all of the RAM and ROM areas). Having sorted the memory map, the Fluke revealed a couple of bit errors in the video ram. Replacing the two 2102 ram chips sorted the problem.
Oh, and the blank white screen – the chip select for the character rom was high, when someone had replaced one of the video ram chips before, they must have damaged the track. Adding a wire link returned everything to normal.