Correction, I have determined my main earthing cable is 10mm2, not 6mm2 as I
said, and the main earth terminal could take a 16mm2, but the CU cannot. I
have plans to upgrade the CU so there is hope I could have 16mm2 main
earthing cables eventually, only I have to stick with 10mm2 for now, unless
I parallel two up.

I have just unscrewed the Bakelite cover where the main cable terminates,
and emerging from the main cable there is a sheathed red core, probably
35mm2 or some such, a skein of smaller twisted black insulated conductors,
which I take to be the neutrals, and 4 bare copper conductors, adding up to
16mm2 probably, going tothe main earth terminal.

I suspect this proves it to be a TN-S system then, as I appear to have a
discrete earth return in the cable.

Andy.