Although you can increase the number of sockets in a room by converting single sockets to doubles, there may be times when you need an extra socket where none exist already. In this case, you'll need to add a spur from a ring circuit. You can either run this from an existing socket, or from a junction box that's connected into the cable run of a suitable ring circuit. The circuit must have RCD protection.

Surface installation means plastic boxes and trunking mounted to the wall

Flush means flat to the wall buried cables in the wall or building fabric .