Discounting the fact that it is diesel (which I have no practical knowledge of). I think your on track with earthing (or ground as we call it). There are 2 primary ground (earthing) points. One tends to be under the hood, close to the battery and fuse box. This is ground for the starter, lights etc. Then there is another ground. On my 07 (American version), it is the passenger side pillar (behind the kickpanel plastic). This ground point is for you ECU, low current lamps (like you dash) etc. On the odd chance that it has corroded or otherwise gotten somewhat dodgy, it can lead to all sorts of interesting phenomena (like screwing with analyzers). That's the only help I can offer. Good luck!
Steeda SRI w/CAI intake. Trubendz SS 2.5" . FR Susp/Struts. Tune by Tom (FocusPower). Rota Grids. SAP rear spoiler. Custom front air dam, rocker panel bits and Rear valance.
Last of the Hatches #28