Problem starting a Ryobi RCT 2800CB Petrol Strimmer?

This is my second question following the advice from John (many thanx). I followed his advice fully, when I tried to start it a few days ago; switched-on,full throttle, choke on, a few presses on the primer it did turn over but then cut out, so I tried again this time without the choke on, but no luck. I cleaned the oxide off the sparkplug, tried again but this time it would'nt turnover at-all. Last night I replaced the sparkplug with a brand new one, I asked the retailer the recommended spark -gap, and was told it should be correct as sold. To check if the HT lead was working I attached the sparkplug to the coil inside the rubber connecting elbow, gave a gentle tug on the pullcord and could feel the current passinig through. I then gave the pullcord a really hard tug but could'nt see any spark jump the gap. Before any of the above it was always really difficult to start but once it warmed-up, it was then very easy, infact it was working just a few weeks ago. Just to recap; the petrol/oil ratio is correct, theres no contamination in the fuel tank, the original sparkplug was a Champion RDJ7Y the new replacement is a NGK BPMR7A WHICH I was advised would suffice. PLEASE HELP! THANX in advance.