aboard to the forum MacCullum. I'm sure that we will be able to help you with your issue and here is what I would suggest you start with unless the other members have other suggestions as well.
I would suggest that you give the sprocket a good soaking of WD40 or something similar for a few hours before attempting to remove it. The if you have a brass dolly you could place it in one of the teeth and give it a hard tap (not hard enough to damage the tooth).
Then if you have a steel bar drill a couple of holes in it so that you can put two bolts in (the bar) so that they go into the tooth holes neatly. Once you have done that you should be able to undo the sprocket if it has not come loose when you tap it with the brass dolly of course.
Try this and let us know how you go.
