Steve,
I'd noticed that checkbox previously, but had misinterpreted its likely behaviour and steered well clear.
But, with it enabled, I'm certainly getting a bit closer to where I wanted to be, though it still leaves PermitRootLogin enabled globally. I had intended to disable Root from the WAN.
(It may well be that KeyAuthenticationOnly, when no-one has the key, is as hacker-proof as a total prohibition on RootLogin would be anyway).
I am, however, beginning to suspect that pfSense may not be correctly honouring MATHES of host addresses when given in the form "192.168.1.0/24" (as specified in the man pages you pointed me at) whilst it does accept "192.168.1.*". This may explain some, if not most, of my earlier confusion.
More news when I've done some more testing…