After a bit of Googling it seems that 4mm^2 cable might be used in a
radial installation but I disconnected the wire between two sockets in the
room and they both still worked - does this allow me to conclude that I
have a ring mains? Also, 4mm^2 cable doesn't seem to be available from
B&Q, etc. - the next size up is 6mm^2.

Maybe it was a radial at some point, then was taken back to the CU and made
into a ring?

Are there any markings on the cable?

Sparks...