My house is old & I've just found out I only have a 30 fuse on my 9.5 shower

Unless the 30amp fuse has been blowing regularly its hard to see how this would cause the switch, which is a separate component to burn out. It may well be fused at 30amp because of the cable size. A 9.5kw shower really requires a 10mm cable with a 50amp switch and 40/45amp circuit breaker. Similarly the shower circuit must be RCD protected for safety. From the little information provided it sounds like you have an older style consumer unit and it may not have RCD protection, as such your best bet would be to get an electrician to call round and either quote for a complete upgrade or at least get an electrical installation condition report carried out to see what other issues maybe present.

As stated above, a 30amp fuse controlling a 9.5kw shower(42amps) is actually making this safer, and would not expect the this the reason a switch burnt out, the main reason the switches burn out, is due to poor installation, and the fact most people switch the shower off using the shower switch, not the shower its self, this means you switching the shower on load, and cause arching on of the switch contactors, which in return oxides the contacts of the switch and this builds up and makes it harder for electric to flow, increasing the heat, hence why you get the burning effect, unfortunately its a common thing with showers, and a £10 switch every other year, may be the best answer.