The pics in the "how to" are gone so have uploaded them again here with the original comments.
OK, I had a go with another spare key I had.
First I tried holding the rear metal part in the vice and pulling on the long key part with pliers... no luck. Then tried locking mole grips onto the key part and it still wouldn't budge even with a lot of force.
So a slight change of plan... locked the long key part in the vice and hit the rear metal part with a BIG hammer and after 2 good hits it separated them OK.
A few pics to show how it locks together:
as you can see the blue plastic bit goes around the long key part (3rd pic), so it's not possible to remove the plastic bit without breaking it unless the metal key part is removed first.