Try a freeing agent such as WD40. Spray plenty on, all around the joint, leave it for 20 - 30 minutes and then try to separate it.
I hope this helps!

...and then ask a man with strong hands to give it a go. You will have loosened it first, of course!!

If you get a pair of old tights, cut the legs off, tie one leg loosely round each part. Put a strong stick in the loop, twist tightly and then use one to turn left and one right.

I would try WD40 as Ian has said and if it does not free it take it back. These are not cheap and should last and your does not look that old?

hi thanks to all