It looks like Pak Choi, but as i didn't plant that til Sunday just gone, it's not Pak Choi.
It has a short tap root, very short - it sits on the earth basically, with a small root.

This bit is growing in the middle of the round bit in the garden. I planted Sunflowers, Lavender, Nasturtiums, Lily of the Valley and Carnations there - and i#m pretty sure its none of those.
It doesn't seem to look like a weed, and if it is - where has it come from?!