Can anyone recommend a good lightweight power sander? I'm going to be using it for sanding paint off a ceiling, so I'll be using it above head height and want something fairly light.

I've used a mouse sander for that sort of thing. The one I've got is a Bosch, which I picked up in a sale at B&Q for about 35 quid. It's pretty good for taking off old paint, etc and it's got a dust catcher on the back which catches some of the dust, though you still need a good face mask when using it.

It's very lightweight though it's not all that easy to hold, you have to grasp it in the palm of your hand - would be better to find one with some sort of handle.

 Black and Decker make one like that, as reviewed here.

 However, it looks to me as if that model may have been discontinued.

Homebase in Penge had them in when I was there on Saturday. I think they're a bit heavier than the Bosch ones though.

Just bought a light weight sander to get old crumbling paint of bedroom walls (just moved house - no basecoat!), found this
Orbital Sander on Pentagon Direct.

In comparison to others mentioned here, believe its good value, as for 1 have used it, it is under £30, it has good amount of oscillations per minute at 16,000. Also it comes with 10 sheets of sand paper with different grit levels which came in useful to get good finish.  You get free delivery too as cost of it is over £25 (only just at £25.06p).   

Not sure about comparison in weight as not been into B&Q or Homebase, I think it is light enough though if using on ceilings.