An update: I've decided not to use Phylum's design since I can't get my head around it 'cause I'm too stupid. However:
@Phylum: Your efforts weren't in vain since you showed me that there's more than one way of doing this. Also I took onboard your suggestion of using a XOR gate instead - So that's what I've done. It's a lot more compact and does exactly what your design did without the need to alter the shape of my construction or the orientation of the pistons (even though I haven't really got a clue how a XOR gate works... lol. I just copied a design and it does just that). So thanks incredibly much.
Now all I need to do is replicate this 6 times. In Survival. Yay. Maybe I'll draw up some diagrams of each level so that people can help me.