How cam I fix my Mountfield SP530 rotary lawnmower?

My Mountfield SP530 has been stood unused for the best part of a year due to wanting a new gearbox, which I've now sorted. The problem is that now it won't start. There is resistance on the pull cord and the engine does turn over freely, However, once the spark plug is removed the resistance isn't there. I put the plug back in but it's still the same.