Mr Cholmondeley-Warner wrote: [good points]
Try the following. While driving in a straight line, look to the left (not necessarily at right angles, perhaps just at the centre of the windscreen below the RVM). Do this for 2 seconds, then look back at the road and see if you're still on the course you thought you were.
Now take your left hand off the wheel and put it in the free space in the centre of the car as well. Again, hold for two seconds, then check your course on the road.
Now do the same but make some signs for 2 seconds, go back and check your course relative to the white lines on the road.
If you continue to drive in an absolutely straight line without thinking about it, you're safe. Any deviation, and we've shown that even your basic control of the car's direction is affected. This doesn't even acknowledge any loss of mental focus that may be accompanying the distraction - loss of peripheral vision to the right, loss of attention on events in front, etc.
I'll do that when I can and report back on the results.