Stuart, the next process to to check each of the basic functions of an internal combustion engine: fuel, spark, compression. First, remove the spark plug, reconnect the ignition lead to it, and clamp the metal body of the spark plug to a metal part of the engine - usually one of the cylinder head cooling fins is the most convenient. Then set the speed control in the start position, and pull the starter handle briskly whilst closely observing the gap between the two electrodes, shown circled in yellow:
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(Ignore the apparatus holding the spark plug - that is what you do to make it easy to clamp the spark plug to the cylinder head, if you do this fairly often and don't want to have to mess about each time.)

As you pull the starter, you should see an evenly-spaced (in time) series of blue sparks across between the spark gap circled in yellow. If you do not (you see no sparks or red sparks), we have found a good reason why your engine does not start.

If the sparks are blue and regular, the next step while you have the spark plug out, is to put a teaspoon-full of petrol into the spark plug hole in the engine, then immediately reinstall and tighten the spark plug, connect the plug lead, and pull the starter. The engine should start and run briefly. If it stops after a couple of seconds, we've learned that your carburetor is not supplying fuel to the engine. If it starts and keeps running, we have reason to suspect that your starting problem is intermittent, or is just something simple like you forgot to use the choke. If it doesn't start, we will have to suspect a more serious mechanical fault in the engine, but we won't accept that as a conclusion until we repeat all of the tests a couple of times.

Please go through that series of tests and post the results. A picture or two would be helpful.