also I found the suzz/gain and reverb controls all affect each other- even with a previous carlsbro amp which was fine. A bit unpredictable, you could say

Yeah, mine does that too... ..and mine cost £20!! In the case of my cobra 90, this is probably due to the lack of proper power amp section. When the signal is sent to the power amp, there are smoothing caps and god knows what else, preparing the signal before it goes to the output. I don't have any of that, so it sounds like dirt.

Crazily I happen to be working on the same amplifier; although mine is the mk1 (no green on the control panel ).
You can do me a favour, I need images of how the power amp links to the control panel pcb, and the power amp wiring generally, colours, etc.
. Regarding your amp, I notice the reverb output cable to pin 8 is missing (it's the reverb ground wire- there are 4 connections to the pcb from reverb tank, in out shield shield). That would account for the noise floor you're getting when reverb turned up. Those accutronics reverb tanks , they corrode bad, so flaky sig/ground connections...