In article
Phil L wrote:
snip
I 'guessed' that his shower cable wasn't big enough, so did a lot of other
people.

There's no evidence of that in this thread - you're on your own.

Considering a 10mm cable would be preferred to a 6mm cable if it's a modern
shower (over 9.5kw), that was my best guess, you obviously didn't like it
and he never came back to tell us any further details, so I'll stick with
that, you're the one blustering and fussing thast it /may/ be 'adequate'
but what if it's not? - neither of us know this and so it's better to err on
the side of caution?

That may be the case when specifying new work, but with an existing
situation it is often better to take the "if it ain't broke don't fix
it" approach. Especially as "fixing" the OPs situation might involve
considerable expense and/or end up with it more dangerous than it is at
the moment.