NapoleonD wrote:So, i need some rcd switch on the inside of the house that will let me isolate the power supply, as well as being a failsafe circuit breaker. Eg daft teenagers plugging a speaker into the wall and balancing it on the side of the tub.
This conversation seems to have gone off track a bit and is confusing two different issues.
You do need an RCD. Under the current regulations, they are required for all sockets. So as you had this installed by a professional electrician, I'd expect that there is one installed. The easiest way to recognise an RCD is that they have a "test" button. They may be built into the socket, or in the consumer unit ("fuse board"). If it's in the consumer unit, it may cover the whole house, a group of circuits, or a single circuit. If you've found one but aren't sure what it covers, press the "test" button and see what turns off. The "whole house" option isn't generally recommended nowadays - while it's safe, tripping it will turn off all power to the house, including all the lights going out - but it's a big job to change so don't worry too much about it.
You shouldn't have more than one RCD for the same circuit. It doesn't add any safety, and there's no way to know which one will trip when there's a fault. It's not unsafe though, so don't worry if you do have too many. So, assuming that you do have an RCD somewhere that covers this circuit, you can ignore the RCD issue.
The isolator isn't so much a safety device - it just stops the outside socket being used when you don't want it to. It's useful to have an isolator inside - it stops anyone stealing power, and it lets you isolate the socket if it's faulty (e.g. gets water in). You want a double-pole switch - the specific one you linked to is, but a lot of fused switches are single pole. A single pole switch will let you switch the socket off to stop power theft but won't necessarily help with a wet socket. It doesn't need to be fused, though. If it's a single socket, a 20A switch like this -
https://www.screwfix.com/p/p/85073 - is more than adequate. For a double socket, that's still OK by the regs, but personally I'd be inclined to go for something bigger like this -
https://www.screwfix.com/p/p/451pf . If the outside socket is on a ring (i.e. there's more than one cable to the socket), you won't be able to add an isolator without making some wiring changes - probably best to get the electrician back in that case.