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You could concrete in some metal posts to put the treated timber on - that would lift everything off of the ground.
Obviously, their own solution is composite too so shouldn't rot: https://www.millboard.co.uk/products/subframe-accessories/plas-pro/
In order for the screws to be invisible, you need to use their ones: https://www.millboard.co.uk/products/subframe-accessories/fixings/. The screws do disappear into the top layer of the material and leave a small hole about 1-2mm across which you can see if you're looking for it but otherwise you'd never notice.
If you're ever south east, you're welcome to come and take a look.
Ah Champion timber is where I have found it, looks like they are still the cheapest.
I guess my one worry with the wooden is obviously it will rot eventually, so was pondering using concrete posts and then bolting treated timber to that, so the bit not on the ground is not wood at all.
I have a few samples coming in the post, got the Millboard in front of me, just their cheapest Enhanced Grain Brushed Basalt, it does look nice that's for sure.
Sorry one more Q: they say you can just drill screws through it and they heal over and aren't visible, is this actually true? Some of the other systems have channels on the sides for hidden fittings.