I am so glad to have got to the bottom of this one, really I am!

The DASH2 loom is powered from the tacho block connector. The 12v and GND pins are utilised from here as they are unused since the RPM counter is replaced by the DASH2 anyway. I had followed this as per the instructions.

The reason why I thought the DASH2 5v reference was stable at 4.96v regardless of engine running or not was because I was measuring with a multimeter between the 5v reference and chassis ground. This would obviously bypass any earthing problems I had and would not show up the problem.

What I should have done instead was measured between 5v reference and the DASH2 earth on the tacho block connector. When I did this it would give 4.96v, then 5.04v if I turned the lights on, 5.07v with main beam on, 5.12v with the engine running, and increase more as the load on the battery did.

So the 5v isn't stable, yet it is when measured across itself and chassis ground. So their is either an earth problem, or a ground loop where other components find the DASH2 an easier path to earth than the path they are given, which is why the 5v feed is being increased, and this is the problem. If the 5v reference is not stable then this will throw the sensors out.

I removed the DASH2 GND cable from the tacho feed and ran it directly into the chassis and then remeasured. Battery on, 4.96v, headlights, 4.96v, main beam, 4.96v, engine running, 4.96v!

I literally punched the air with happiness knowing I had found the problem - it was the earth cable in the tacho that was feeding back current all along!!!

All sensors worked perfectly except for the fuel sensor which still altered in value with load, so I took the fuel sender straight to chassis ground too and it's now completely stable too. I'm extremely happy with myself for working this out.

I have to say how highly I think of Lorne and his team at Race Technology for helping me solve this issue even though it was clear very early on that it was down to an installation issue. 10/10 for factory support.