My mower hit a steel reinforceing rod while mowing and the timing is out,won't start. I checked the piston, conrod, valves etc and all OK. My mate was helping me and removed sump and took out cam shaft. How do I set the camshaft in the correct position?

There will be a mark on the cam gear and the crank shaft gear line these up ,but sounds like you sheared the flywheel key , that will put it out of time and make it jerk back when you try to start .
Luck

Thank you DG, will check flywheel key as well.

You might have gone to a lot of trouble for no reason. Most camshafts have alignment marks or one mark and use a keyway for the other. Good luck with that. While you have the camshaft out, check the crank shaft to make sure it turns smoothly. Sometimes they bend, if it is, you have a portable lawn ornament. Not sure about this one but most have a soft steel key in the flywheel for timing alignment and shearing in case you hit a steel rod. Check that, if even slightly damaged replace. I hit a tree stump twice and lost the key both times and the blade collar once. It still runs, but gives your hands a nice massage while you mow.