Many newer types of BS1363 outlets are moving away from the earth-pin-operated shutter mechanism in favor of a type which requires simultaneous and equal pressure on the line and neutral shutters.
The latter have a shaped edge (hard to describe), and inserting meter probes into these means pushing the probes simultaneously on the top left of one shutter and the bottom right of the other (or bottom left and top right).
The latest designs apparently require pressure on all three points to open, although I've not come across any of these yet.
