On Friday, 15 May 2020 12:02:33 UTC+1, JoeJoe wrote:
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.

If the post is well and truly rotten below ground, you could remove that rotten wood. Then you should have a fence-post shaped hole. OK, might not fit perfectly but a little bit of fettling might be enough. If needed use stones, sand, wedges to stabilise the post and make it vertical.