807 cuts out

[Dazzler101]

Bought an '07 2.0 HDI about 6 months ago, it has been really good no problems. Last Friday whilst driving it just cut out. Completely. No lights on dash, all out. Turned ignition off and on again, normal dash lights on again but would not start. left it for 2-3 mins, tried again and it started. Managed to turn around, got about 1/2 mile and it did again. Waited and it started again and it went further before cutting out again and managed to get home after another few stops (I was close to home when this happened). I hooked it up to my lexia and no fault codes shown. A friend of ours is an AA man and he suggested main power relay. If so, where would I find it. Also, I am not sure but the immobiliser may have been bypassed, we only use the non plip key as the plip one was broken, and on the lexia when I went into new parts it came up 'warning ecu ........... RHR. Any ideas gratefully received. I went on a huge voyage of discovery with a C8 last year before it's eventual termination and I really dont want a repeat

[wheeler]

The non plip key still has an immobiliser chip inside it to deactivate the immobiliser.
The main relay is an integral part of the under bonnet fusebox.
First thing i would check if there is a total loss of electrics is the battery connections then the main earths after that.