Your improvised voltmeter looks like it was made for the job; nice one Petemate.
Could I ask where you sourced it and which make / model it is?
Having not done much related to vehicle electrics; is access and tapping in to the cigarette lighter supply relatively straight forward?
Finally, though I realise you added some sort of filter to the lens I'm not completely following the "Used a print from images 'shades of grey'" bit.
Here is one of the sources:-
http://www.ebay.co.uk/itm/170670726769?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649(sorry - don't know how to do links!)
The place I got mine from have no more, but just do a search for digital voltmeters and there are lots available. Make sure you get one 'no power needed' - this means you don't have to mess around with the panel light feed from the power socket.
I made the mount from the corner of an old mains socket box from the workshop. The feed was fairly straightforward; I'll post some pics later (my lad has borrowed the car to go to work.....) Regarding the shades of grey - I did a search for 'grey' in images and got this:-
I then carefully prised open the tags securing the meter to the housing it came in, and there was a celluloid lens on the front of it. Using this as a template, I tried, starting with the lightest, shades cut from a print of the image until I was happy with the reduction of brightness, then reassembled - voila.
Granted, the car has a warning should the charge fail, but (from my experiences with my cars over the years) I do like to see how the charge rate is behaving. It shows some variations eg when the aircon is on, and during normal (80-ish mph) running with lights on, registers fairly steady at 14.3-14.4. One can see, when starting, how the rate rises once the glowplugs have cut off again.
I am convinced that the old alternator had not been charging at full rate, due to the slightly slower rate that the starter was spinning the engine over in the few days prior to the failure; bear in mind, that AFAIK, it only takes a few volts to put the charge light out and just because it has gone out doesn't mean all is in order!
I mounted it where it is so I can just keep an eye on it when I want, also I didn't want to mess about with the dash anywhere else. Works for me!
Many thanks for the kind comments.
Pete