Any idea what the brightest light available with a decent rack-mount (I have three holes arranged horizontally at the back of my rack which I guess is a standard mounting)? I found this roundup which although US-biased was very helpful http://bicycles.blogoverflow.com/2012/03/tail-light-review/. From that selection the Cateye Rapid 5 with optional rackmount sounds like the best option to me.

Alternatively I have an Exposure Flare (which is ace) but I can’t mount it to the seatpost as it might be obscured by the stuff on the rack. Anyone have any joy purchasing or making a decent rack mount for this light?

Loads of mounting options here:-
http://www.sjscycles.co.uk/rear-light-brackets-dept784/

I use my Holy Hand Grenade on the back of my rack, you can get a rack mount for most lights I would imagine

I made a mount for a smart 1/2 watt jobby.

thanks, looks like a HHG with appropriate mount will fit the bill perfectly!

cateye tl ld1100 fits straight onto a standard reflector bracket (same layout as most racks) it even comes with the correct screw.

Cateye also make a special plastic fitting that screws into those holes and accepts any of their lights with the square slide-on fitting. Wiggle usually sell them, about £4 IIRC
They work very well.