Seen this before, if you remove the other bolts and push the DV upwards at an angle it snaps the lug off the DV and you can remove the DV and grip the bolt with mole grips. Block the DV hole with some cloth to avoid anything going in when removing the stuck bolt.
Try all other options first before resorting to snapping it, but if its an old DV you are binning anyway its not a big problem.
If you have some torx bits try these as well as they often fit into a rounded off hex bolt.