Hi guys,
I've finally got it running like a dream! Thanks for all of your help. I really appreciate you taking the time to help me.

I tried the trick of putting petrol directly into the inlet port. The mower wouldn't fire. So I decided to look at the spark again. Again I saw a spark, i tested it a few more times and i noticed that the spark was only really happening at the fastest turn of the pull. So took the HT Leed out of the plug cover, then held it close to the engine block. I noticed I could get a spark at really low rovolutions of the engine that way. The spark plug is brand new! So i regapped it to a small gap of 0.02mm. Then i noticed i was getting a spark much more consistantly. Happy days. The engine start and ran on the first pull of the starter rope!

I've tried it several times since and the mower is starting and running like a dream!

In the end i've;
1. Replaced the HT lead, which had an intermittent break
2. Replaced the spark plug
3. Regapped the plug
4. Cleaned out the exhaust ports

A combination of these has my mower running!

Thanks again guys, you've been a great help.