David Hansen wrote:

10.8kW at 230V comes to a little under 47A, according to Mr Gates'
calculator and my fingers, so a 50A MCB sounds likely.

No - check the rating plate and/or instructions for the shower and
you'll almost certainly find that the 10.8 kW rating applies at 240 V.
For design purposes at 230 V the current will be close to 43 A and the
output around 9.9 kW - so a 45 A fuse or breaker will be fine.

What earthing system does your house have?
I have no idea. I thought all houses were earthed the same

Certainly not - see
http://www.wiki.diyfaq.org.uk/index....Earthing_Types

Remember that instant electric showers involve the water people
stand in passing over a bare uninsulated heating element.

As already mentioned by others, this is not the case. The elements are
insulated. RCD protection is now de rigueur and would be
incompatible with an uninsulated element.

You suplimentary equipotential bonding will need to be up to scratch
unless you plan to implement 17th edition style 30mA RCD protection for
all circuits in the shower room.
Meaning?

It is a good idea for you to get someone else to design and fit the
installation. That is the best advice you will receive here.

And a Part P application (building notice) is required of course...

--
Andy