On Wednesday, 24 June 2015 10:59:29 UTC+1, Jim x321x wrote:
Can someone tell me if I can get away with a 45A MCB for a 8.5kW shower
without endagering life and limb? The manual tells me to use a 40A one, but
I just happen to have a 45A one. I ask, because those things aren't cheap.


Probably, as you're not relying on the MCB for overload protection (as the load is fixed and unlikely to draw a higher than rated current), merely for short circuit in the event of fault.

What is the cable size an mounting method.?

Owain