If interested, here's the low down on my frustrating day...
First thanks Hoolander for all the information. Unfortunately right now I'm not in a position to be able to do those things as a) I simply don't have the time (am finishing assignments for the last year of a postgrad course) and b) this is my only form of transport at the moment and as such need to get it fixed asap, which unfortunately means taking it to a shop. ...Had this happened any other time then I would have spent a while looking into all those other possibilities as I would like to become much more self reliant to fix things and troubleshoot them.
So, four and a half hours later and a cut finger, of trying to replace a spark plug, it's done. And unsuccessful.
I finally managed to get the old (15 month) spark plug out with the wrench socket. The end of it did look black. Not oily but definately black all round the tip of the thread. ...off to Halfords I went, bought the nearest one that was recommended to me (called a NGK C7HSA when the one recommended for agility 125 is apparently an CR7HSA - the guy said the R stands for resistive and won't make any difference)
Got home, put the new one in, wasn't too sure about how to secure the rubber cap back into the whole and within a few seconds had broken the cable off that feeds into this rubber cap. (as you'll see in the attached pictures).
While I was there holding a lead with just wires sticking out of it and the new spark plug in it's socket, I thought I would hold them together and try starting the moped. No luck, not even a spark.
I tried reputting the cable back into the rubber cap and plugging this back into the spark plug socket. Still no luck. (It was a case of screwing the exposed cable into the cap to connect that back up again)
Now unfortunately I'll have to take it to a shop, as I presume it wasn't the spark plug at fault, even though the old one was black. That means I'll be cycling across town for the next few days as I even find a gap to take it to the dealer (or my local car mecanic, wonder if he will do it?)
The attached pictures show my old spark plug and the cable that I had accidently snapped out of the rubber plug socket.
Still interested in your insight to the situation... perhaps there was something I still did wrong with regards to the new spark plug?
Thanks.