Right, I had a go at fixing this engine this evening and heres wat happened:
Tested the plugs - they had a nice bright spark, so I can rule them out.
I tried fresh fuel (my tanks are plastic BTW) but this didn't have an affect. Took the fuel filter in the engine out and checked it. There were a few odd bits of crap in it but nothing major.
Took the carberttor off. Removed various covers and things but there was no evidence of any crap here (saying that I'm no expert in this field; I assume I was looking in the right places......where is the jet/what does it look like??). Put it back on again but still suffered from the same problem.
I'm out of ideas now. Any more suggestions before I book it into a Mariner man?