Well, I've totally re-done it, and I think it looks much nicer. I took the dirt out, as it was looking nasty, smelling nasty and apparently killing my plants - even the ludwigia had turned black and rotten where it was under the substrate. I have been aerating it, but apparently not enough.
So, now I have black gravel, the ludwigia (new stuff) in the centre, and a java moss carpet. I might add some hornwort as a floater once it has all settled. Pics to come. :)
Can anyone recommend a good light?