Just to add to the confusion here... my car died post IoM trip while en route to Lancaster for Phil to sort out the Centa spline issue. I had assumed it was the regulator but in the rain / Liverpool traffic I had not noticed any warning light (or monitored the charge). Power loss was total (all electrics, engine).
After a while on the hard shoulder the battery had recovered enough to run the hazard lights for a short while.
Phil reports the regulator (original) is working OK with a slightly low (<14v) output, and the battery charged up fine on the Li CTEK (starting the engine a few times without a run).
The battery is a Li based one, so can't blame Exide or lead acid issues! The mystery continues...
