Ross, the first thing to try to settle is whether it is spark, fuel or compression. I suggest you try the B&S compression test (remove plug lead, then flip the blade plate backwards toward the compression stroke, and see if it bounces off compression). It isn't all that likely to be the problem, but it is possible. Second test: try to check whether it actually has spark when it doesn't run. Now that it won't start at all, that should be easy to check.
If you have spark and compression, you should check that the flywheel key has not sheared. All of those checks should be done first because then the only thing left is fuel, and that is a bit more complicated to check, and more likely to give uncertain results. The first fuel check is, while it is refusing to start at all, squirt a small amount of petrol into the cylinder or inlet pipe and see if it will start and run briefly.
If you get to the end of that set of tests, you should know definitely which system is at fault. When you have that information, let's talk some more.
