(Mr MC posting)
20 quid for Cree LED torch, 2 sets of rechargeable batteries and quick/smart charger from far-eastern ebayer. Then improvise a helmet mount with velcro straps. Helmet mount is better than a bar mount, especially on techie stuff, as it shines where you look not where the bars are pointing. If you already run head torches then stick it on your bar.
I have a Cateye EL700 tripleshot which is a gucci >200 quid rrp light, and bought the pikeytorch as a helmet mount to accompany it. Before I got the chance to use it me n MC night rode, and as her lights were flat I knocked up a bar mount for the torch. It is brighter and has far more penetration than my tripleshot, and you get over an hour from each pair of batteries.
Just ordered some more batteries for a few quid so we will have several hours worth of stonking light for less than 30 quid. And I might be buying another one for me as MC seems to have claimed it…
http://cgi.ebay.co.uk/Flashlight-CREE-Q5-LED-UV-U-Torch-3-7V-CHARGER-Holster_W0QQitemZ260360124790QQcmdZViewItemQQptZUK_SportsLeisure_Camping_LightsLanternsTorches?hash=item3c9eac3976
ignore the UV bit, I cant find the torch without it and its no use, but its no hindrance either.