Doesn't seem to matter what I add to it, it always dries up. And when it has, it leaves a massively unbalanced lump of latex in the wheel. If I add enough stuff to stop it drying as much, it never seals (obviously). Thoughts (currently back on tubes AGAIN)?

are you adding anti-freeze to your ghetto solution? That can precipitate a reaction and cause the latex to solidify like that.

I found that the JRA gunk dried up into a lump quite quickly but Stan's or Joe's don't suffer

spend some money on Joe's or Stans – both last 6 months plus for me with and even when they do dry it tends to be evenly all over the tyre wall rather than a lump at one point.