"Dave H" wrote

There is some flat, twin-core and earth cable in my house that is 20mm
wide, overall. Would that be 16 sq.mm cable or 10 sq.mm? It's the
standard white-covered pre-2006 stuff.

It is important that you check the conductor size itself on these issues.
Some modern cable has significantly less insulation than was applied to the
same "size" just 20 years ago!
I recently replaced some damaged 6mm cooker cable and was so surprised at
the change in over-insulation size that I had to mic the conductors up to be
sure.

Phil