Non-start on Flymo XLT 250?

Flymo XLT 250.it ran well a month ago but now will not start. Filters checked and cleaned. Petrol is being pumped through. Good compression, Plenty of sparks from the plug when removed, earthed, and the engine turned over. I wonder if the plug won't start whilst fitted and under compression. I only suggest that because I had a similar problem with an old Velocette m/c magneto.