...and it turns out it was a problem with my own equipment! I've just replaced my old Zoom X4 with a nice shiny new Linksys AG241 and my download speeds have shot up from 0.1 Mbps to 4.0 Mbps. So there we go, not everything in the world is plusnet's fault ;-)
And just to even things out my LLU connection appears to be on a go-slow - 50 -> 100Kbs on a 2Mbs connection, and now I know who pinched my bandwidth... give it back at once I say...