Tried it the other week and it worked great. All you need is the iron on about medium heat, place the tea towel over the dent in the sail and a book underneath. Place the iron on the towel only and leave for about 10 to 15 secs. Keep checking your progress. If there is crease it will not come out, but a dent should.
on a slightly related topic - Does anyone know any tips for removing old (other peoples) sail numbers from 2nd hand sails.
I feel like a sail thief looking for some tips