Hi Tony, if you do the maths, a load of 8.5kw works out to roughly be 36amps. This is the reason for the fuse blowing. The most important thing to remember, is the fuse is there to protect the cable from overheating and melting. So for you to be able to upgrade your fuse to a 40amp, you need to make sure the cable size is sufficient. Without looking at the length of run from the consumer unit to the shower, it would be hard to tell, but a 6mm cable would be ok.