James wrote:
Thanks for the advice. The single sockets were not wired to the boxes from
the socket earth terminal, maybe its some new regulation thing. The single
sockets have been in since the house was built in the mid 70's.

Yes, it's a 'new' recommended practice - but not that new! Thinking
changed around the mid80s, AFAIR. Previously, it was considered that the
backbox would get reasonably well earthed through the faceplate screws,
which rested in little brass cups which were solidly joined to the
metalwork of the faceplate, which included the 'wiring accessory's earth
terminal. ('Wiring accessory' is electrician-speak for switch, socket,
fused connection unit, etc.)

However, there were a few documented cases where corrosion between the
screw and the backbox increased the resistance between backbox and earth
to the point where a fault current flowed which was large enough to
cause notable heating - I think even a fire in a couple of cases - but
too low to blow the fuse/MCB. This was more likely where you had the
unfortunate combination of only one of the faceplate screws having the
brass cup and fixing to faceplate metalwork mentioned above, and that
being the side with the adjustable lug of the metal back box. (Most?
backboxes have one fixed threaded lug on one side, the other one having
some up-and-down play in it to allow the accessory to end up all
luvverly horizontal like even if the backbox is marginally skew whiff.)

Since this was publicised by the IEE and in the odd 'How To Do Sensible
Wiring' book, practice has changed to prefer a flying-lead connection
from the accessory earth terminal to the back-box's earth terminal - and
backenboxen aren allen maden withen integralen earthen terminalen. But
you'll still find older ones - like the Original Poster's - wot don't
have such a terminal. Taking a flylead and poking it hopefully into a
spare mounting hole doesn't really make a solid enough contact; it's
worth at least using a short self-tapper to secure that lead (you may
need a few mm depth of the material behind the box drilling out with a
handy masonry drill).

HTH - Stefek