Posted: 05/07/2012 at 13:48
OK, so here's the upshot: I plugged my hands in, and carefully tried to extract the plant... I needed to use lts of strength and what came out was an UGG=Unidentified Growing Green-thing. Sort of a GM hybrid of woody radish/parsnip, withleaves which indeed remind of rocket, but with an unpleasant smell. Result, I pulled out the three similar UGG that were occupying a big chunk of the raised bed, and gave more room to the growing cabbage. Meanwhile, I "discovered" some nicely growing beetroot, looking like beetroot (at least for its leaves) that I couldn't see before because of the invasive "Dalek".
Thanks to all of you for your help!