Alistair,
Go to Wal-Mart, HD, Sears, whereever and pick up a Power Inverter. I have one that is capable of handling 750w. It has three outlets on it. It requires a marine/car battery to power it. Marine is better, it has a deeper charge.
The marine battery is 12v DC. You will connect the power inverter to it (cables will be included) and then you plug in your heater, powerhead or two and you're set to go. Should keep you for a few hours.
If it goes longer than that, you might have to run your car and hook the inverter up to that until you can recharge the marine battery.
Either way, that's the least expensive, effective way to do it.
BTW, You have a PM.
Russ