Well, I'm pretty sure I've found out what the problem was (and it's kinda embarrassing).
Turns out my chain was just really dirty. It was caked in a thick black tar-like substance. I don't know whether it was dirt, grease, oil or whatever, but its all been cleaned now. I used WD40 the previous night and today I used some grease-remover and some chain lubricant.
And so after months of owning my first motorbike and weeks of having to endure a terrible clunking sound whilst riding, I finally learnt the importance of regular chain maintenance.
Guess I'm going to have to start checking my chain every week / fortnight as Senna suggested. I'd best get checking my oil levels while I'm at it...
