Existing cable into electric shower unit useable?

Just checked the cable into electric shower from mains to see whether 6mm or 10mm in order to upgrade the old shower unit. The cable is, I believe '10mm T+E', but it is black/red not brown/blue. Also the grey outer sleeving is nearer to 15mm wide (sounds silly but is this classed as 10mm?)
Is this existing cable still ok to use?

Competent Person Scheme Operators