luxeon led dimmer circuit

I've built the following circuit on breadboard as a dimmer circuit for 3
luxeon III Star ultrabright LEDs:

http://www.niftybits.ukfsn.org/electronics/luxeon-dimmer.png

any comments welcome. It does work at the moment, but when the battery is
low, the maximum current drops from 0.9 A to 0.7 A, and the op-amp swings
near to the top rail. The one thing I'm thinking of changing before I
solder it up is to change the 1 ohm sense resistor for a lower value
(maybe 0.5 ohm) so that the transistor has a bit more headroom to work
with, which should cure this I think.

The LEDs are /very/ bright by the way - 3 of them are enough to light a
small room well enough to read by. The light is a bit unkind on the eyes,
but not too bad. 3 at full current are about equivalent to an 11 Watt
energy saving fluorescent light bulb, which puts them in roughly the same
bracket as far as efficiency goes.

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