Hi Nickgriff
First remove the spark plugs, (just in case it is hydraulic locking) and try to turn engine over with kill cord removed, if this doesn't work then.
Check the conections on the battery first, if they are clean and tight find start relay and with a peice of thick wire (or a screwdriver) short out the two big terminals (don't turn the ignition on) if this turns the engine over okay its the relay if it doesn't it could be the starter motor.
Hope this helps