Oops...

After the desoldering of the z80 went without incident, I expected the socketed replacement to go without incident too. I soldered the socket quickly and cleanly into place, replaced the CPU and hey presto...  a choice of either red, magenta or black border with flashing attributes.

Just in case I destroyed the original CPU during the desoldering process, I had already purchased a spare and fitting this gives me additional black and white bars. The internal speaker is buzzing and nothing is getting unduly hot, I have also checked fairly thoroughly for shorts and solder bridges.

I suspect the problem is due to a poor solder joint on the top side of the board, I'll check them thoroughly before crying for help. There is no response to the reset button and sometimes the pattern changes after being left powered on for a minute or two, hopefully I haven't damaged anything else while working.

EDIT: Information regarding this specific fault is contained in the Spectrum Repair Guide, looks like I need to check each and every individual PCB track going to the socket for continuity.