I finally got my .28" led voltage/time/temperature module installed, now the rain has stopped.
http://www.ebay.co.uk/itm/4004616846...%3AMEBIDX%3AIT
It seems to work well, has an accurate pulse width temp sensor, a back up battery for the clock and a 2 button key pad. TBH I don't need the clock so the battery and extra button are not needed, however maybe I could scrap the big clock for the unsightly mpg meter. Anyway it was only £4!
On the down side there is no brightness control, and the switches are slow to respond. It tends to use too many digits, so I'm seeing resolution to 2 decimal places on temp and voltage, just because it has a clock! A "v" would have been handy. Less is more!
I had to extend most of the wiring but the ignition 12v signal identified above was useful for "on power". I still had to wire the gnd and voltage sense wires directly to battery to get the error <0.1v, although this is not permanent until I get a suitable in-line fuse.