I've just done a little googling and there appears to be no definitive answer I can find. Even a driving instructors forum posted completely differing views.
A government site just said get out of the car and use the emergency phones ... nothing else.
What if I had a can of bio in the back and ran out, could I top up and drive off?
It would appear that you can tow a car with a tow rope, off a motorway if it's broken down on the motorway, but you cannot use a motorway to tow a car that broke down elsewhere.
When I had head problems with the Disco, I carried a cubie of water in the back. I stopped on the M25 to top it up and the motorway patrol turned up. We had a nice chat on the hard shoulder after I filled it up and I went on my way. Even letting them know I knew it was liable to happen due to the head problem, they didn't have an issue with it.
Be nice to know a definitive answer.