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from "Dave Plowman (News)" contains these words:

In article ,
Mike P mike@askme wrote:
The circuit cable is larger. It has 3 coiled earth wires for a start.
I spurred off with a 30amp junction box using 2.5mm.
Why should the cable all be the same size?

It sounds like you've got old cable called 7/0.29. There will be 7 strands
of 0.29" twisted together for the live and neutral and 3 of 0.29" for the
earth. That is the old standard ring main cable replaced by single strand
2.5mm TW&E about 40 years ago when such things went metric. If it has PVC
insulation it should still be fit for further service. If earlier it could
be rubber and likely to need replacing.

Hmm.............. While it was general to use 7/.036 for ring main work
before metrication, that would have had 7/.029 for the ecc.

If the OP's cable has three strands to the ecc, then it was probably
indeed 7/.029 which would have 3/.036 for the ecc.
(IEE Regs, 13th Edition)