What caused play in my freehub? I was getting my road bike out earlier and heard a clunking sound when I dropped the back wheel. I repacked the bearings in it ~200 miles ago, and the wobble felt like bearing play. However when I removed the wheel it was the cassette that wobbled, and when I removed the cassette the freehub moved from side to side inside the wheel hub. It was fine before I repacked the hub.

What on earth could have caused the problem?? Perhaps degreaser somehow got into the freehub and wore something down...

UPDATE

I followed you advice mikes, although my 10mm hex key was too small for the nut. Furthermore, when i grasped the freehub body to inspect the amount of play, it came off in my hand!



The bearing you can see aren't the 3/16" hub bearings but lots of tiny little ones that were packed inside the freehub. I believe the other half of the freehub, the part attached to the hub by a hex key, is still bolted onto the wheel. 

The wheel is part of a bike from decathlon, which is still covered under their parts warranty. Would this damage be caused by a defective freehub or have I broken it somehow during my maintenance regime?
It is possible that the freehub has become loose from the hub. Remove the axle and bearings from the hub. Insert a the correct size allen wrench (I believe a10mm) into the non drive side. Tighten the retaining nut. If hub continues to spin you may need to use two wrenches. One inserted in the free hub  and one to tighten the retaining nut.