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The posts and concrete boards wouldn't be less than useless, they are a much better bet than the normal fix using sleepers mounted on timber posts. I have seen many horrors used as retaining walls, 1" planks and paving slabs on end are amongst my favourites. There is a special place in my heart for chicken wire on bamboo stakes.
The half length concrete boards with concrete posts are a good idea, the biggest problem is the leverage of the built up ground against the posts. Double the posts would help by, well, it isn't difficult to work out how much. However, the gravel boards are reinforced, so you will need an angle grinder with a stone blade and a metal blade to cut them down.
ColinTheBald-
Thanks again for the input. I had read a few random posts from other forums taking about paving slabs on end. Seeing as I've broken those before just by standing on them, that was out. One of the collapsed fences here is chicken wire and (now) rotten battens. Just pulling that out with the tangled ivy means that couldn't be further from an option.
stevo_com
Halving the space between posts is not a bad shout. Just means three extra posts (and holes) but I was worried about the 6ft span of the gravel board.