jkn wrote:

I'm looking for some freestanding light duty shelving
The shelves themselves would be of size approx. 300mm deep by 2m wide, at
possibly-adjustable heights. Max height will be 1.5m or so.
The 'difficult' thing is that I want the shelves to be supported in the middle
of the (300mm) edges, not at the four corners.

Some variation of "Gondola" shelving, perhaps?

https://google.co.uk/search?tbm=isch&q=wire+grid+gondola+shelving