Now that I have found a bit of momentum I am keen to keep it going, so I am just doing a quick post to cover a little progress.
Needing 12v for the screen and 5v for the Pi and arduino and any other devices I may connect in future, I have always wondered the best way to power everything from one input; is it best to pump 12v in and step down for the 5v devices or vice versa?
Anyhow I was digging through my bits and bobs and I found something ideal for this (see picture below). A variable power module.
I ordered two of these little beasties from Ebay many moons ago. They were cheap; and it turns out thy were cheap because they were in kit form. Lacking confidence in soldering so many parts in (read: lazy) I ended up not using them. They've come in handy now!
The boards themselves have a barrel jack or screw terminals for 12v in, and then has a series of screw terminals on board; a set for 12v out, a set for 5v out, and one set for a variable voltage, adjustable onboard using a tiny flathead screwdriver.
Picture (from a new angle for @Stefan Lochbrunner's enjoyment :-) ):
The above picture shows The Pi (Just a B+ for now) on the lower level, and the screen driver on the top deck, and to the right of it, my power module.
Over the next few days I will hopefully get this wired together, but first i need to get the screw holes and tabs adjusted so the case all fits together nicely, then create a frame/bezel for the screen to make it fit the hole better, and adjust the holes to the side of the screen so the screen driver buttons can be accessed.