Hmmm, my explanation of the timing procedure probably wasnt the best, read the haynes manual, it will make more sense.

The sensor isnt the only thing that can go faulty no, there is also the dizzy cap and rotor arm that can fail, but you can tell a lot by looking at them, and cheaper to replace.

Did you test to see if theres a spark straight from the coil as you said was described in the haynes? If there isnt a spark from that lead on its own, the problem isnt the cap/rotor arm as its not even in the equation, leaving what DazC said then, either a wiring (Unlikely), ignition ecu, or dizzy sensor fault. Personally, im putting my money on the dizzy sensor, a very common thing on these.

Mine is on its way out, occasionally it miss-fires and back fires out the exhaust very loudly, its funny at the moment as it still drives, but ill probably when i brake down.

I'm waiting for DazRS to get back to me about where the sensors come from, and as for fitting, if i said removing the old sensor is harder (which isnt actually hard) than the new one is to fit, would that grab you by the goulies?