Problem with a de-humidifier is they suck moisture out of the air, and if you have drauhts, you are letting air in from outside, so defeating the object.
Is the building attatched to the house, and if not how far away is it?
i have recently added a radiator in the garage, I was extending the heating anyway, and i picked up an old rad at work for nowt. My garage is 21 x 15, with about 3 feet square of usable space, and the rad really does its' bit to help. It takes the chill off, a few weeks ago it was -2 outside, and 12 inside. But that is with as many draughts plugged as I can find, and the ceiling fully boarded over with Jablite. Even cutting panels for the doors makes a difference.
The way I see it, if the Doris is warm watching Eastenders on ice up the jungle, I may as well be warm doing my hobby, and the heating is working no harder to do the garage.
How big is your hovel, and what construction. When you say utility, is there a tumble drier in there by any chance?