I've done a bit more digging. 18mm plug sockets are often labelled as 12mm sockets, (at least in the UK - not sure if it's an international thing) I think because 12mm is the thread diameter.
As suggested above, a normal plug socket will not fit, as the wall is too thick.
Triumph sell a special plug socket for the Speedmaster (Part Number 2300107-T0301), which I've ordered. The cost is £13, compared with about £3 for an off the shelf plug socket.
