Hello My name is Ivor, I have just come on line and saw your question.
I have had this problem with a strimmer. I found that if you can take off the guard. Hold the spindle with some pliers so that it doesnt spin. Make sure you wear gloves in order to stop damage to yourself. Then with the spindle held unwind the the line in the opposite direction to the normal spin. If that does not work try heating the line and break off sections of the line with some pointed pliers.