After coasting to the hard shoulder, it restarted no problem.
I've done 60 miles since, and it's not done it again. I'm sure I didn't knock the key with my knee, and the weather conditions weren't bad when it happened.
I've checked all the main earths under the bonnet and the battery connections - all fine.
What else should I check? I do 80miles on the M62 every day, so I don't want it happening when I'm in the fast lane.
Thanks