g'day again Gold33, that happens when not assembled right. It sometimes takes a few go's to get it free, also if fitted without or wrong gasket (washer) under the driven end.
I have seen the worm drive on ebay and think SRM do one and I think someone on the forum said you can get new gears, but they are usually ok if not on a high iron diet.
If your not in too much hurry you can send it down to me for inspection and repair if possible. I'm near Ulladulla, my email is in my profile.
Cheers