ECU earth issues

Hello all - my TD5 90 first started to fail to start (if that makes sense!) a few months back. Based on posts on the Zone, the earth connections to the transfer box were cleaned and re-fitted with a covering of vaseline. The front end of the injector loom was also replaced as the red connector to the ECU was full of oil - the oil has pretty much stopped re-appearing now but the connector was cleaned again a couple of weeks ago so should not be a problem. Went to use the Landy yesterday, turned the key and...temperature gauge up to max, no ECU of glowplug lights in the dash! Left it for a bit and tried again - that time it started, but I don't trust it now. I have read elsewhere that an additional fix is to run a new earth for the ECU - so my question is..from where to where? From the fastening in the bottom of the seat box for the black lead from the ECU connector, direct to the negative terminal of the battery...or is there a better option? Thanks as always for the advice!

Here's a fix that was shown to me by a Land Rover specialist

Drill a small hole as shown in the photo. put a self tapping screw into the hole, and tighten.

'Simples' or what! Thanks! Presumably this just provides another ground route between the seat box and chassis via the floors or seat belt mount?