This has now happened 3 times and is rather disconcerting. My left hand front rotor is coming loose. I have tightened it as far as is possible and tightened the lock bolt till I almost broke my allen key but it has come loose again. Took almost a month of daily driving but it still happened.
What am I doing wrong if anything and how can I fix it?
Am thinking of using a crown nut and drilling a hole through the stub axle.
What have others done?

Are you sure it's the spindle nut loosening off and not a wheel bearing slowly disintegrating?
That will cause play in the hub too cos unless the thread is dodgy on the spindle or the nut it's pretty rare they loosen once the allen bolt is locked down.

You may need to clearance the split in the nut, is there still a gap after you have tightened it down?

Maybe a new nut might solve the problem.

are the bearings seated correctly

is the thread on the lock nut and spindle OK

Do the bearings match the stubs correctly?

Check all dimensions.

Threads are ok, nut was so tight I couldn't adjust it without loosening the locking bolt. Also after tightening the rotor rubs on the backing plate when cornering. So I am assuming that the wheel bearing is stuffed. WIll be buying some new ones tomorrow abd hopefully fit them after work.

Did you check the sizes. Wouldn't be the first time someone fitted the normal disc bearings to the one year only 68 stub that has a smaller dia. shaft at the inner bearing......

Have just replaced the bearings this arvo. After tightening again I noticed a rubbing sound from the rotor and the backing plate. After pulling the rotor off there was slight scoring damage to the rotor and the backing plate. So I guess bearings were stuffed. Hopefully this fixes it.