Hi There,
I see you are in the UK, therefore a definate candidate is the Deben Tracer, and "ordinary" bulb type light that fits on top of your scope and is good for anything out to 100yds, which is more than adequate for "bunny bashing" with a pre charged air rifle. The battery pack is seperate and is in your pocket or on a belt. This light will last approx 3 hrs (if on constantly) with the included batt pack, again plenty for a nights hunting, you can also get the red lens filter for them that just clips on the outside. Very light (in weight) and a good allround performer, I have used one of these for many years and cant fault it, I even use it on my rimfire for bunnies and it has never let me down.
I think a lot of the lights discussed on here are not as applicable to the use you have, as the Tracer is, but there are always alternatives and exceptions. The Lightforce 140 is another I use on my centrefire rifles, much brighter and with longer throw (lights foxes up at 400yds) but much more expensive than the Tracer and probably over the top for your use.
In my experience for rabbiting, you need a light that will throw a tight beam at 50 - 80 yds with little or no sidespill and runtime of approx 3hrs.
Lightforce
here
Deben Tracer
here
Hope this helps a little.
Regards
Lee