Ok so, as an experiment, I just fired up the BD on my XBMC setup and used the per-movie settings to turn the brightness down from the default 50% to 46% and it really makes the difference! Blacks are deep and black again instead of grey and murky, some of the visual effects that previously looked extremely dated now look seamless and the 'suiting-up' scene where Batman slowly raises his head is far more impressive as his eyes emerge from darkness rather than just getting a little brighter.

I can't say with any certainty at all that this simple change has returned the film to how it originally looked but it definitely looks far better and reminds me more of how I remember feeling while watching the movie as a kid. I'd love to know how dark the original print actually was and whether only certain scenes were lightened for home video, but until we have a good source then this will do just fine. :)