A couple of things from experience here in the U.S. Radishes (as well as other root crops) don't like a lot of fertilizer... to over fertilize or being planted in freshly fertilized soil will result in large leaves and no roots.
Additionally, while radishes like about 5 to 6 hours of sunlight, they really should be planted in an area that gets some shade during the daytime hours.
Lastly, we've experimented with grandad's old maxim of planting according to the Moon's phases. Surprisingly, our records indicate somethings going on with this ancient thought process. However, (you'll need to consult Google for explanation of terms) thequote[waning gibbous[i] cycle is thought to be great for planting root crops if following the [i]Synodic] method. We're in the midst of the Waning Gibbous period for this month, by he way...
If interested, see here:
http://www.the-gardeners-calendar.co.uk/moon_planting.asp
Luck!