so swapping the fuse out is sufficient,
FFS.
You will also (I can virtually guarantee) need to replace the cable for the shower circuit
FFS Helpful!
I have not measured the cable running up to the bathroom looks substantial
So how do you know that swapping the fuse is sufficient?
You don't know what the size of the cable is, so how can you know what it's current carrying capacity is?
Will measure it tonight but would say it looks more like 10mm
Do you know how it's installed, and whether any insulation or grouping factors need to be applied to derate it's capacity?
When you say how its installed, please explain, insulation no it runs along the inside wall of the garage, up through the floor into the bathroom which is on the back corner of the house, I would say total cable length is less than 10m, seeing as this cable along with a 30a fuse is currently running a 8.5kw shower I dont see how capacity will be an issue, but please let me have your thoughts
Do you know how long it is, and whether the voltage drop will be within limits and whether its R1+R2 will be low enough to give you an acceptable Zs for a 40A device?
As above circa 10m
so swapping the fuse out is sufficient,
Almost certainly not, and absolutely definitely not without checking the suitability of the circuit first.
What about swapping out the MCB for a higher ampage?
I was more concerned with having to swap the switch on the board
What switch?