cable size and mcb for shower install

What size cable and mcb would be required for an 8.5kw electric shower,
cable run from switch to cu is about 8-10m,was think 10mm cable and a 40amp mcb

Depends how the cable is installed, i.e. on or through any thermal insulation, with other cables, the ambient temp of the run etc.

8.5kW = 8500watts

8500/230 = 36amps so a 40amp breaker is OK

Assuming the cable is in free air then 6mm is OK

However, an 8.5kW shower is quite small by todays standards and you may want to upgrade at some stage. Personally, I would agree with you and install a 10mm cable which would cover most large domestic showers.

What type of consumer unit do you have? Can you post a picture as some are not suitable for this added load. Also, you should have a 30mA RCD in circuit.

Search Part P in the Wiki before you start so you know where you stand.