Dave Plowman wrote in message ...

[4mm^2]

If this is now the 'legal' size, does it mean the 1mm earth built in
to most lighting TW&E is not satisfactory?

No, not at all. 4 mm^2 is the minimum size for an unprotected CPC which

- is not an integral part of a cable, or
- is not formed from metal conduit, trunking, etc., or
- is not contained in an enclosure formed by a wiring system.

[Reg. 543-01-01]

It's analogous to the 4mm^2 minimum size for supplementary bonding in
bathrooms: size is dictated by the need for mechanical, rather than
electrical, robustness.

--
Andy