You can still buy the dedicated "clock connector" here, which has been around for many years:

[Linked Image from mkelectric.co.uk]

The front plug section has three very small flat blades which fit the outlet, and contains a cartridge fuse to BS646. They're ceramic-bodied fuses similar to the BS1362 fuse which is found in the 13A plug, but considerably smaller and available only in sizes from 1 through 5 amps.