You are lucky because I'm Dutch and can read it, I emigrated to New Zealand 20 years ago.
Get the REL26 which can switch 7 Ampères at 230 Volts.
Your clock speaker may be supplied with ac so a small capacitor may be required before the 100 ohm resistor. A bit of trial and error may be required here.
The BC 547 is a standard NPN transistor and act like a switch. When about 0.6 volts is applied to its base it will turn on and the relay will pull in and closes the contacts. The 100 ohm resistor is there to limit the base current.
Be sure that the diode is the correct way around, otherwise you blow up your transistor.
If the relay draws more than 100 mA use a 2N2222 transistor which can handle a bit more current.
These transistors are cheap so a couple of spares would'n do any harm.
Powersupply: get a plugpack 12 volts dc output 300 mA , 500 mA or 1 Amp.
These can be used for other projects as well.
Good luck with your project.
Raymond