How do I fix my jcb plt 25f strimmer?

Did a great strimming job the other day at home, as the cord ran out, I tried to release the bottom plastic wide nut that holds it together and it just keeps spinning and doesn't come off!

I remember it just took a couple of turns previously and detached to let me get at the cord, now it just turns as though the thread is knackered. It's only been used 6 times over the 18 months I've had it. Im unable to get at the inside without wrecking it. Any ideas?