David Hansen wrote:

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

Nope. They're just glorified electric kettles.

The water
is in direct contact with the electricity. That means the shower
must be installed properly, or it is likely to be dangerous to use.

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.

It's not rocket science though, is it? If you don't know, why not just say
so instead of being so fecking patronising? This is a DIY group unless you
had forgotten. The *rules* may be a little complex (and I admit to not
knowing them myself) but I'm sure anyone with a bit more eloquence and
patience than you could explain them simply enough.

To the OP, this site may help.

http://www.diydoctor.org.uk/projects...ngbathroom.htm

Tim