As you say, the disc is held on by only the wheel nuts. It really is a simple job to change discs and pads on these - even I can do it! Just jack it up, take the wheel off and have a push-and-pull at all the bits. When the discs and pads were changed, the chap doing it should have taken out the two slide pins (take out the rubber plug covers over the two pins and use an Allen key, can't remember the size, down the hole. Do 'em one at a time so it doesn't all come apart) and cleaned them so that the caliper slides freely. 'Appen garages won't do that because it takes too long, which is why you're better off doing the job yourself. And you'll need a 13mm socket to undo the two caliper fixing bolts. The rattle you report isn't a common fault, so only getting down and dirty is going to find out what's wrong.