Okay so I just bought a signature Slash wah pedal and every time I turn the distortion on it up to about half way it cuts out. I have a 240V Randall amp, is there something wrong with the pedal or do I just need a bigger amp?
Sounds like a bad gain pot. And no, you don't need a bigger amp. Basically a bad part in the pedal (probably the dial that you turn the distortion up on) is cutting the signal out.

(Also, amp "sizes" are referred to by how many watts they are, not volts. All amps in the UK run on 230/240V.)

If you just bought it you should return it.