Well, not really a repair as it was so simple. Game booted consistently to the same garbage. The usually suggests the CPU is doing something (as the garbage would be random each time if it wasn’t) but goes wrong because of RAM or other IC’s. However, replacing the socketed Z80 CPU fixed it! Easy fixes are always good.
I must admit I made a mistake when wiring the pcb at one point (DK3 style edge connector to old style DK block connectors) and ran monitor blue to sync and vice versa. The monitor actually managed to sync to the blue output – and then you can see the sync represented on blue on screen. The long blue line is really the vertical blank sync, and the short blue lines are the horizontal sync pulses – they are just all out of place of course. So if your game looks like this – check wiring!