Coloured vertical lines with white border - diagnosis / advice?

[Distinct0983]

My recently acquired Spectrum 48K was working well until... turned it on and was greeted with these static coloured lines, with a white border (looks grey due to a cheap hdmi adapter).

Not sure what caused it but I suspect something power related - I plugged the original power brick in at the plug while already connected to the spectrum, which is when the issue started. I also had a DivMMC connected.

I checked the voltages that various posts seemed to suggest,
ICs 13-6 all had -4.57V on 1, 12.07V on 8, 4.96V on 9, and 0.0V on 16.
D11 at 2.61V
R55 at 55.3V
Across the three legs by the power socket (the regulator?) it is 10.75V between the left and middle, and 4.97V between the middle and right pin.

Only things I've changed since getting it is to replace the z80 due to a bad m1 line and attempt to reposition the spider mod as outlined here (https://www.sinclairzxworld.com/viewtopic.php?t=5408). It was working correctly for a while after that though I wouldn't be surprised if my work is the cause of an issue.

Any and all advice gratefully received!