I agree Elaine, it does look like a japanese radish.
On reflection, it could quite easily be a Mustard - I've found ones coming up from my emptied-out sprout trays with a white root quite similar. So are the leaves and they are bordering on spiky.
What did the plant look like?
Hmm, thanks James and Elaine. No idea what to do with it, but I'll experiment - my girls will love that!!
The plant, Andrew, looked very much like a slightly spiky Rocket...
Wow, I had exactly the same thing in my summer green manure mix from Green Harvest seeds.
Daikon is milder than red radish. Go ahead and taste it! I never knew they grew well here. Gonna have to try them now!
You can gratethem for a salad/slaw. they make a lot of juice when you do so. Matches great with soy and sesame.