Stefek Zaba wrote:

Since this was publicised by the IEE

Any reference to when and where "this was publicised by the IEE" and
quasi-officially recommended? ICBW but I don't think the NICEIC
technical manual mandates it if the mounting box is not touchable.

One situation in which this link /is/ mandated is where the incoming
earth (CPC) arrives at the box via a metal conduit, cable armour or
sheath. Then the earth connection to the accessory must be made via a
separate wire link from an earth terminal on the box to the earth
terminal of the accessory. [Reg. 543-02-07]

Of course the 'converta' sockets (WWIANF) mentioned might not have any
earth path to the box via the mounting screws, in which case the link is
a good idea.

To connect to old boxes with no earth terminal I recommend a short self
tapping screw (say No. 8) in conjunction with an insulated crimp ring
terminal on the end of the wire.

--
Andy