I've looked in the FAQ's etc but am puzzled whether my electricity supply
has TN-S or TN-C-S
earthing. I thought that TN-S had the earth supplied by the incoming cable
sheath, and that TN-C-S
had the earth supplied by tapping an earth off the neutral core as it enters
the service head thingy.

Now then, my system has an incoming cable with a central red insulated core
surrounded by 16 black insulated 2.5mm2 wires. The red core is used as live,
12 of the 16 black cores are used as neutral, and four of the black cores
have had their insulation stripped and they have been directed to an earth
terminal.

Although at face value this means that my neutral line is being used to
provide an earth, the 4 earth cores are not connected to neutral in my
house, possibly they are only connected back at the substation - which would
happen to an earthed cable sheath anyway.

Any thoughts as to which earthing method best describes my system?

Andy.