In reply to d508934:
> So how straight forward is this for an averagely competent DIYer, but currently with no fencing tools or fence erecting experience? Just one wooden post that is knackered/rotten at the bottom and now held up by adjacent panels.
> If set in concrete how easy to remove the old broken bits and slot in a new one? Presumably easier if one of those metal spikes was used.
Very easy. If the base of the post is rotten and set in concrete you can remove the rotten timber down into the concrete base using a steel bar. You then have a slot left in the concrete into which you can push a new post, with the aid of few whacks from a sledge, and maybe some washing-up liquid as a lubricant on the post-end. However, you need exactly the same size post (in section) as before, and if the existing post is planed, you also need the new one to be planed as the extra few millimetres for a standard sawn section will mean it won't fit.
Of course it'll rot off again within 5 years or so. But it's a quick fix, and i've done multiple times on my boundary fences.
If the concrete base starts rotating as you dig out the rotten timber, then dig out the whole lot and do the job properly.