Try using a suction dent puller. These are the same things as are used to carry plate glass. You can buy a set of two 11.5 cm and two 5.5 cm pullers for less than £8.00 including postage. Have a look on e-bay. If the dent was caused by something sharp the metal may have stretched in which case you'll need to fill it the same way you would a car. If all else fails get a large fridge magnet to cover it.
Nigel
Nigel Benson, September 2007