Not a Dracaena, that has spirally arranged leaves, not opposite like this plant. There's a Euphorbia that looks like this, not sure which species.
I agree; it looks like Gopher Spurge, Euphorbia lathyris http://davesgarden.com/pf/go/55261/index.html HTH Chris
Thanks all. That's the one. I'm a total beginner so have been trying to ages to find what it is. FYI, this plant is growing in London UK, clay soil. Thanks again.
I had it invading from a neighbour's garden, and the neighbour also got rid of it after it started taking over - so watch out!
Think some parts, the sap is one, of euphorbia can be quite poisonous or at least irritating to the skin and eyes.