I too have a suspected problem with my voltage stabiliser... but first, here's what it looks like and where it lives on my Mantis.

It is screwed onto the back of the centre dashboard section between the two leftmost gauges. It is the unit with the '3797' label on the top and one stumpy screwdriver (philips if I remember correctly) will remove it, the mounting screw being hidden at the bottom of the unit going through a protruding tang which is soldered/brazed onto the back (just visible in the photo).
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The labels on the unit are Caerbont Automotive BR1307-00

A replacement is available from the Demon-Tweeks catalogue but they say that the delivery time can be up to 6 weeks as they are only made in batches and it depends if there are any on the shelves ready for delivery or if a new batch needs manufacturing. I ordered mine yesterday.

My symptoms, which have exhibited themselves for about 18 months, are a voltmeter which has a mind of its own, giving an indication between 'deep red' (less than 10v) to somewhere in the 14v+ range. At first my thoughts went to high electrical load but after experimenting with different conditions (headlights, fans, steady cruise at differing speeds and all combinations of  anything I could think of) I realised that there seemed to be no common factors except that the problem arose once the engine was thoroughly warm after a few miles of running. So, alternator methinks.... The alternator output checked out at all engine speeds from idle to 3000rpm (not prepared to go steady state revving at anything higher on the drive) as did the voltage at the battery terminals. So, the process of elimination has pointed me to the voltage stabiliser, especially when I discovered this thread.

I've also replaced the battery but that was because it was letting me down by not holding its charge for more than about 3 days. As this has only happened in the last three weeks or so and I don't think it forms part of this particular problem.

I'll let you know what, if any, difference it has made when it arrives.  If it doesn't do the trick then I'm less than £20 quid out of pocket but back to getting splinters in my fingers from another bout of head scratching! 

edited to correct manufacturer's name.  

Try to be the person that your dog thinks you are.....

Edited 2 times by Captain Speaking May 26 11 12:16 PM.