Seemed that a Gas powered station at Little Barford near Bedford tripped out and took a windfarm or two with it this caused the mains frequency to go low, below the nomiunal 50 hertz, that is and the system sort of protected itself.
Some have speculted that it was a test for the days post brexist when power outages will be the new nomal;(
Excellent site here shows whats being used and where its coming from.
www.gridwatch.templar.co.uk/