Yet another TD5 earthing issue!

Found an old thread from 2013 on this matter but figured it would be better to post anew rather than add to the old one! Anyway, I had the same problem with my 54 plate TD5 90 - intermittent (very) failure to start with the tell-tale 'no glow-pug or ECU lights' and the temperature gauge flicking to full deflection at the first key position. A year or so ago, after a tip from another zoner, I put a self-tapper through the the seat box to body mount beside the drivers seat which seemed to cure the problem but it has recently returned (ironically on the day it was due to be MOT'd...which did not happen as a result!). For various reasons, I am not able to get under the Landy and work through all the earth connections at the moment - so was contemplating a temporary fix so that, at the very least, I can get it to the MOT station. So, after that very long introduction my question is - if I make up a temporary/emergency earth cable to take the ECU earth directly to the battery negative (routing it inside the cab), would it be safer to disconnect the ECU earth wire from the terminal in the bottom of the seat box and connect that to the emergency earth cable rather than connect the temporary earth to the seat box terminal? Thinking is that there might be a risk of the light, temporary cable being overloaded by whatever else might try to earth through the seat box as a result of poor connections elsewhere. BTW, the injector loom has been replaced and the red connector to the ECU cleaned multiple times until the residual oil syphon dried up...so that should not be a contributor. Thanks in advance.