First of all, what battery pack are you using (technology, voltage, and capacity)? Your application sounds similar (but not identical) to one of my "back burner" projects.
I assume you're using the Fatman as a constant-voltage source, then using the Rat Shack pot as an adjustable ballast resistor. This is a BAD THING - I'd doubt the ability of the pot to dissipate enough power (remember, you're using only a portion of the resistor, so all your heat is dissipated in a small piece of the pot's resistive band).
A better approach would be to use a constant-current source - there are circuits available online that show how to use an LM317 adjustable voltage regulator as a constant current source. This reduces the battery-to-lamp voltage drop (your voltage regulator has a minimum input-to-output drop, and there's also the drop across your ballast resistor), so you reduce the "headroom" needed between your lamp string and the battery voltage (also, LEDs are current-driven devices - if you're running them near their maximum ratings, it's better to feed them the current they want and let the forward voltage settle wherever it lands than to try to find a ballast resistor which will drop the "excess" voltage at the rated current). You mention 700 mA, which is consistent with running a single Luxeon 3 (somewhat underdriven), so I assume you're running them in series. This is good from a battery life standpoint, since the second one is using "free" power (the power to feed it would otherwise be dissipated as heat in your driver circuit if you only had a single Luxeon).
With 2 series-connected Luxeons, the forward voltage would be approximately 7 volts not counting the regulator, so I'm assuming you're running either 9 volts (6 Alkaline), 9.6 volts (8 NiCd/NiMH), or 12 volts (8 Alkaline, 10 NiCd/NiMH, or 6 lead-acid).
If you're running off a 12 volt pack, have you considered going to 3 Luxeons? The reason I ask is that Luxeon makes a triple-lens optic (fits setups designed for a standard 50mm halogen) in 3 different beam spreads (IIRC, they range from 10 degree to 45 degree), and there are flashlights that use 3 Luxeons (one by the manufactuer of the light in the "cheap Costco Luxeon 2xAA light" thread, but it uses Luxeon 1s, and others by various makers that use Luxeon 3s). By using a commercial flashlight head (assuming it's from a waterproof flashlight), you'd save yourself a lot of trouble with sealing your headlamp (necessary for caving). A quick check
here showed a pair (4xC, 4xD for power) of triple-Luxeon3 lights (both built by Elektrolumens) - a technically-simple (but pricey) solution would be to "repackage" one of these to have the stock head unit mounted to the front of your helmet, and the battery pack mounted either elsewhere on the helmet, or on your belt. Even though they run off 6 volts (driver circuit would be easier to "homebrew" if you have 12 volts, but by starting with one of these you don't need to roll your own), they have decent battery life (review listed 12 hours for the 4xD model).
If you're interested in my solution (still in the conceptual stages, uses 12V SLA as power source), here it is: 3 series-connected Luxeons (I'm planning on using Luxeon 1s, but with extra heat sinking and a different resistor it should work just as well with Luxeon 3s) and the Luxeon triple-lens assembly. As a current regulator, use an LM317 to provide the base current for a power transistor in common-emitter configuration (the load, in this case the string of 3 Luxeons, is between B+ and the collector) - you don't have enough "headroom" to use the 317 on its own as a current regulator. You'd need to juggle the resistor value to get your desired drive current depending on the "beta" of the transistor you use - note that the transistor would definitely need heat sinking (the case on a power transistor is electrically "live" - like the "slug" on a bare Luxeon emitter, but unlike the aluminum plate on a mounted Star, so you'd need a mica insulator and a nonconductive bushing on the mounting screw, but electronics supply places usually carry these), and it wouldn't hurt to do this for the 317 as well (again, the case is "live"). If you're "homebrewing", get yourself a few 10 ohm 10 watt resistors (fairly standard value, so they should be easy to find), and check the voltage and current delivered to the load when running first a single resistor, then a parallel-connected pair, in place of the string of 3 Luxeon 3s - the resistors are
FAR cheaper, and will withstand a
LOT more abuse.
