Mortimer wrote:

The thing is that the concrete is well established at each place there
is a fence post, I don't fancy having to excavate it all out hence my
idea of enlarging the exsisting hole and re-using the solid ballast of
the concrete to support a new post.If I use well treated posts and
keep then free of loose soil they should last OK shouldn't they?

Why not use the existing posts (after cutting of the rotten bottoms)
with concrete repair spurs, which are made specifically for this
purpose, set into the enlarged holes in the concrete?

http://www.andertonconcrete.co.uk/An...iscPrecast.htm