a 4 x 4 treated wood post that was in place for around 15 years has
rotted and needs replacing.

It is a 2.4m one - 60cm under ground, set using a couple of bags of
postcrete. Unfortunately the post has rotted right at the ground level.

Once I free the post from the rest of the fence I am certain that I will
be able to simply snap it at the point where it meets the concrete.

Is there an easy way to remove the lump of concrete so that a new post
can be easily set in its place without having to dig a very large hole
around it and pull it out?

Any suggestions will be appreciated.

PS: I do not have access to any heavy machinery.