Notanumber wrote: Its a tricky one because the fault is inconsistent.

As part of your fault finding I would fist reccommend a cheap digital voltage gauge and wiring it ideally to come on with the accessory position of the ignition switch (radio feed) .  There are several types available from Ebay and actually they dont look bad as a permanent fixture, about the same diameter as a cigarette lighter socket. 
That way you will be able to see if there has been a bit of a drop overnight but just not enough of a drop to prevent starting.

You can get voltage meters the can just plug into the ciggy lighter socket, saving a bit of buggerising about.
But I agree with the idea of first checking for wiring insulation damage!

Read More...