Tom Martin is correct, however i need to add, your shower is required to have RCD protection in line and from what I can gather from your description it doesn't
I strongly recommend you to get an electrician and have this remedied
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.
I agree no is the simple answer, to be honest you should not really still have fuses to supply bathroom electrics.
If that fuse is blowing ask yourself why. A 30 amp fuse blows at a lot higher than 30a. So changing it to a 40a will cause damage.
No. Is the simple answer. The fuse rating is designed to protect the maximum current the installed cable can take. If you increase the fuse rating you create very possible fire risk. The size of the cable and the installation method rate which size fuse you need. If a 30amp has been installed please keep this and seek help from a qualified electrician as to whether this can be upgraded or not.