There is no way on God's earth I would be using 6mm cable for a 9kw electric shower - An electrician friend of mine recommends 10mm otehrwise you will end up heating up more than the water.
The current load on 9kw -39.1 amps so your breaker size will need increased ( an RCBO ) would be ideal.
Please consult a qualified electrician and not some pratt who thinks they know what they're doing.
A useful guide is pasted below -
http://www.tlc-direct.co.uk/Technical/Charts/V oltageDrop.html
All the best.