TheScullster wrote:

The number 4 in the middle was the concern - 4mm cable?

Pass. The conductors of 6 mm^2 have 7 strands of 1.04 mm dia. (7/1.04).
For 4 mm^2 the stranding is 7/0.85.

Anyway, having confirmed the conductor size at 6mm sq, why are the
over-insulation dimensions so much smaller?

[...]

Anyone got any other ideas?

Permitted tolerance - BS 6004:2000 Table 8 gives the following limits:

CSA Min. overall Max. overall
--- ------------- ------------
4 5.6 x 10.5 mm 7.2 x 13.0 mm
6 6.4 x 12.5 mm 8.0 x 15.0 mm

I would expect that the length of cable removed was about 20 years old.

The 1995 version of the standard has the same sizes. I haven't got any
earlier info, unless you want imperial cable sizes to BS 2004!

--
Andy