Hello,
I'm trying to use an arduino board to control some 240V valves and am desperately struggling with what to use to step up the voltage. Sadly I'm quite inexperienced at electronics so a simple answer would be much appreciated!
Thanks 
Tom
OK... You don't need to "convert" the voltage. As you said, you need to control the higher voltage. You also need to isolate the high voltage from you low-voltage circuit (for safety).
1. Since you are new to electronics, the simplest solution is a solid state relay ([u]example[/u]). Most solid state relays will work directly from the low-voltage low-current Arduino outputs.
2. There are a few mechanical relays that can work at 5V at less than 40mA, but most require slightly more voltage & current to "activate" the coil. So, most regular (mechanical) relays will require a transistor or MOSFET driving the relay coil. [u]This assembly[/u] has some electronics and some relays and is "ready to go".
**3.**With a TRIAC and a special opto-isolator designed to drive a TRIAC, you can essentially build your own solid-state relay. I've done that before, and that's probably how I'd do it again.)
My choice would be option #1.
Don
No disrespect intended but if you are asking this question you first need to do a lot of reading to familiarise yourself with matters electrical before you even think of going near 240 volts.