I had the same problem last week. Luckily my car has good earth points...
What you need is one of these...

Get underneath, clip the black (earth) to the bottom of the gearbox or part of the engine (can't remember where I managed to clip mine on, just make sure it's not the sump)
Clips the red (positive) to the positive point going into the starter motor.
That should give you enough juice to use your fob to unlock the car