On Sunday, June 3, 2018 at 9:23:01 PM UTC+1, Bert Coules wrote:
Richard wrote:

I got the impression Bert was considering simply
to rest gravel boards against his existing slotted posts?

That was certainly my idea, laying them horizontally and relying on the
infill to hold them in place. But it's been suggested that the plastic
posts, however rigid and however far sunk and well secured into the ground,
probably wouldn't be able to take the sideways pressure of the infill.

Depends on the infill. Clay will give you side pressure but compost/loam shouldn't. I'd use my old chum, vermiculite, mixed with the soil. This keeps it light and airy, and a hell of a lot easier to work.