Just a quick update on this...
It seems that the HDMI-to-VGA cable I have (the Amazon neewer one) draws some power from my Pi. Presumably it needs some to make the chip work that converts the signal. I've no idea how much exactly, but it might explain some of the power problems I've been having!
I played with Debian a lot yesterday, fiddling about with configurations and trying various games (Scummvm, Doom, Quake 3, Pingus) all without any issues.
Today I decided to try out OpenELEC and kept having problems with the screen on my monitor blanking out for a second or so and then coming back. Very annoying. I tried switching to a normal HDMI cable and the problem went away, so I concluded that it must be this cable drawing power from my Pi.
Now as I was saying, I was having some power problems previously, even with a genuine Samsung Galaxy S II charger, which I think without checking provides 700ma. That prompted me to switch to a powered USB hub, with the mouse and keyboard plugged into that on two ports and a third port providing power back to the Pi, leaving a fourth port spare.
Having reached the end of my tether with the screen blanking out this evening I considered not running the Pi from the USB hub and instead using the Samsung charger JUST for the Pi and the powered hub for everything else. I decided I didn't really want to do this as it meant having two separate devices plugged in to use my Pi and I was already a bit annoyed at having to use a powered hub.
Then I remembered that I have some 2.5" hard drive enclosures that come with Y-USB cables. The idea is that you plug both USB plugs in and one provides power and data connection and the second just provides additional power for more hungry drives. As I don't actually own a drive that's ever needed this second connector plugged in I decided I could sacrifice one of these cables.
A quick Google search showed that I would still only get 5v of power on the cable (I was initially concerned I would get 10v and blow my Pi!) but estimates on amps varied between 600 to 1000 ma.
I decided to go for it and grabbed a spare micro USB cable and one of these Y-USB cables and got busy. I didn't bother connecting the data wires as this is purely intended for powering the Pi, just connected the red/black wires.
Plugged it in, turned it on and it works perfectly, the screen blanking is gone!
Hopefully this might help out someone else when they're having a frustrating day...