Basec Electrical Cable 600/1000V, BS 5467 ATOM 3 x 6mmAll XLPE.
3 x 6mm Doncaster is 14.9mm o.d
3 x 6mm Eland and Batt are 16.6mm o.d
3x 6mm Cleveland is 15.14 o.d
3 x 10mm is around 19mm
There's nothing close enough in 3 core for it to not be 6mm
The writing is so small that you can barely read it.
The best way is to run you r finger tips and feel the indents of the label and you will need a light to flash onto the cable where I saw the number/sizing.