There’s a recent thread about one of the new 2xXML units with external battery here. Personally I use 18650 cell torches… a charger and UK adapater is £5 from Dealextreme, mounts are a couple of quid, I have an abundance of free cells from old laptops, and the latest LED torches range from £8 to £15 each.
My current set up is:
501b XM-L2 from Lightmalls – £8 for a head torch on this mount
C8 XML-U3 also from Lightmalls – £11 as a bar torch using this mount
but you can now get a C8 XM-L2 for £12 which might be even better (depends on the driver settings)