One of our instructors came up with the following tip to get a knot out of a kite line and today one of our demo Parks came back with a very tight figure of 8 knot on one flying line, so I though this was a good opportunity to try his tip. Tools needed: 2 x paperclips and 2 x pliers. Started by spraying the knot with silicone (wetsuit) spray and then gently tapping the knot on wood base with a small hammer to loosen a bit. Then worked a paperclip into each side of the knot - this took patience and about 15 minutes. Once the paper clips were in place, a couple of tugs on both clips with pliers and voila knot loose. In the pic the knot looks quite loose but I can assure you it was the usual super tight when I started. Not sure how it will work on a single knot.
Indeed, a smooth surface, a hammer and some water-based lubricant is probably the least destructive way to get a knot out or your lines. As g0nz0 says, just keep rotating the knot and tapping it with the hammer. It takes a matter of seconds.