Before I ran mains to my garage I used a 12V gel cell battery (or, you could use a spare motorcycle or car battery) and a 12V florescent tube with bayonet fitting hung from the ceiling. The bayonet fitting means a standard 240V lamp base/holder can be used - search ebay for "12V 20 Watt DC Fluorescent Bayonet" to see the type of light I mean
Assume current of about 2A for one light. So, a 20AH motorcycle battery would theoretically power it constantly for 10 hours. However, in reality you should never fully discharge the battery - so I'd charge it after 5 hours of use (or carry the battery with you and leave on charge whenever not in use).
Alternatively, as others have suggested, LED lighting may be a good option. Again, I'd get a 12V solution and run it from a spare motorcyle/car battery.
