On Friday, 26 June 2015 09:12:55 UTC+1, Jim x321x wrote:
It's a 6mm^ T&E cable that is surface mounted all the way to the 8.5kW
shower. (about 4ft goes through surface-mounted conduit.

6mm clipped direct is ok for about 47A.

The only risk is that someone at a later date might see the 45A MCB and think it's okay to put a higher-rated shower on it without calculating for the 6mm cable size.

I'd leave the 45A MCB in and label accordingly.

Owain