Raised bed against a fence

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.

I do have a sizeable quantity of hardcore-type rocks and rubble which I was hoping to use at the lowest level, topping that with smaller stones and coarser earth and a top layer of better quality soil. Wouldn't that put the majority of the sideways pressure against the bottom foot or so of the fence posts, where presumably they're at the point of their strongest resistance to movement?

Thanks for that, and I could do it for the upper two-thirds, say. But this is a good opportunity to get rid of that hardcore if it would be feasible to use it.

It's not worth killing yourself over. Maybe you'd be better off with some lighter weight wood. It won't last as long and will need more support, but you can do it single handed. Line the wood with DPM so it lasts longer?

I agree wholeheartedly.

I was thinking of plastic tarp or roofing felt. This was my original idea. I can live with the need for extra bracing, and I've found wood (suitably treated and/or clad) to be surprisingly resilient outdoors.

Thanks for the thoughts.

If you avoid clay, I don't think you'll have a problem, especially with the rubble at the base (and obviously drainage holes along the bottom). Treat it as a container i.e. mainly compost/loam, I use 1 part vermiculite to 6 parts compost. Not cheap but worth every penny

You might consider cement board, expensive though

I built myself two raised beds about ten years ago out of "sleepers". Kneee high and about 6ft x 9ft. Then I built two more only 4ft "narrow" with one 10ft long and the other 6ft based on experience.

I use mine for veg. The reach to get at things in the 6ft wide beds isn't doable even with my reach AND I have to kneel to achieve maximum reach (I'm just over 6ft). If I was starting from scratch I'd build to about waist high and possibly higher and then adopt a no dig policy.

There are some recycled material kits available. This one

Don't forget to research soil. You DON'T want anything containing clay. Also think where the load is going to be dumped whilst you move it to the bed. It's quite easy to block your car in, for instance (I remembered just in time!).

I've never used it for anything, indoors or out so I'm really not familiar with it at all: I'll investigate, thanks.

Well wood isn't exactly cheap (though for this I could probably use secondhand/well-weathered if I can find some).

Thanks, Stuart.

Yes, I've seen those, and also similar material in plank form. Rot-proof of course, which is a big bonus, but also considerably pricier than wood if I recall correctly.

Thanks for the soil tip, and also for this:

I'll bear that in mind.