Mira Play (9.5kw) with 13 Amp fuse which blows?

I have just moved into a new house with a new Mira Play (9.5kw) shower installed. The shower switch outside the bathroom also has a small fuse box below it with a plug-type 13 Amp fuse in it. On the medium heat setting, the shower seems to work fine, but on high heat the fuse always blows and the shower won't work. Should a shower be wired up via a 13 Amp fuse like this? There appears to be a dedicated line from the fuse box for the shower. Are there 30 or 40 Amp fuses I can use without danger or is it more complicated than that?