[Arran]

Kirk;
  I always used a clothes iron and a wet paper towel to steam out dents, or a soldering gun tip for small ones, but a rag will work. I don't think that a heat gun will work as well, you want the water to soak into the fibers of the wood and for the water to explode into steam to pop the fibers back out, a heat gun will probably dry it first. It works well on poplar and mahogany, reasonably well on maple, ash, oak, birch, or beech, but only so so on walnut.
Regards
Arran