Update:
Turns out we have 2 'fuseboxes'! The secret fusebox was hiding in the garage, and governs the circuits that had stopped working. It had been tripped.
Of course, I only discovered this after opening up 4 of the sockets on that circuit
It occurs to me that having 2 fuseboxes, but thinking you only have one, could result in a hard lesson.
Thanks for the help anyway, MV.