Do not insert a 40 Amp fuse into the existing circuit.
This much power has to be handled with extreme care. To change the 30 Amp fuse to a 40 Amp fuse could be dangerous and could lead to an electrical fire. Also, certain CU's will only accommodate a fuse of a max rating of 30Amps. If you can post a picture of your CU or give a full description of it, any makers marks, the colour, how many fuses in it, any unused fuse carriers (be our eyes for us), one of us may be able to go a bit further with you.
Upon changing a shower like this it is required, by law, for the person changing it to ensure that the wiring is up to the job, that the CU is - including the presence of an RCD (the regs require all equipment to be installed iaw the manufacturers instructions - and they all say an RCD is required) - and that the bathroom is correctly fitted with earth bonds. It is not a straightforward question, though, at first glance, it might appear so. For example a 6mm cable may be able to safely carry a max load that can vary from 29Amps to 51 Amps depending on how and where it is sited between the CU and the shower. In a normal domestic situation, this range reduces to between 32 and 46Amps. Your 8.5kW shower will need about 37Amps.