I think of it this way...
A fuse is only a way of stopping electrical gear catching fire in a fault. It is not there to protect the person using the equipment. It does go SOME way to doing that but it isn't to be relied on AT ALL for the protection of people.
so if you have a multisocket with only a fuse in, and that fuse is 13amps, then the fuse will blow when 13amps of current is pulled through it. you could power a lot of stuff through that fuse before it blows, or you could fry yourself a few times over before it blows too.
The RCD does what you say. looks at in and out going current and you body becomes a connection to ground, the RCD detects the current loss and switches off the power. Much more effective at saving your life than a fuse.
so the answer is really that you need both. fuses to protect gear from damage, and RCD's to protect you from damage.
The amp will have a mains fuse, in addition to the fused socket (if you're using a fused extension lead). If you had a 13amp fuse in the kettle lead, and a 3amp fuse in the mains input of the amp, you'd expect the 3amp fuse to go well before the 13amp in the kettle lead. if the kettle lead is in an RCD, the RCD could trip before the fuses blow...
in your RCD muiltiway... you'd have 1 fuse in the 3pin plug, then the RCD, the a fuse in 3pin plug after the RCD, then a mains fuse in your amp. the fusing in the multiway's 3pin has to be big enough to allow for equipment to be used at all the outputs. however the RCD doesn't care.
I've used one of those £10 RCD's before for using power tools on sites. It didn't give me a problem but I never checked it out fully. You probably don't want to get scammed and buy a rubbish one.
