Just bought a house which was built in 70s. Most of the things are fine, actually they were renovated recently. But the switch box is old fuse style There is no RCD on the main switch, the only RCD in the house is the one in bathroom's socket, which I think is required by the law. 

So I'm just concerned about the safety of the water heater. It was installed in early 2000's, installation quality looks good, earth connection seems good. 

The question is I would like to installed a RCD device beside the switch, is it legal to do it by myself? I'm quite confident on this small job but don't want to affect my house insurance. 

It looks like to upgrade the old fuse switch box to new MCB with RCD would cost thousands $$$, and as a DIYER one cannot modify anything on the fuse box except for replacing fuse. I'm fine with the old style fuse box as long as I can add some RCD protection to the water/elec appliance.  I understand one can upgrade the existing switch by DIY.

So I'm not gonna touch the switch box. There is a existing switch beside the water heater, Can I add a RCD next to it by myself?