How can I engage forward and reverse gear on a Westwood S1300 lawnmower?

Engine starts and belts are engaged as fan on motor for gearbox is spinning.By pushing the lever forward it should engage the auto forward gear and releasing the foot pedal slowly the forward or reverse gear should engage but does'nt. I have checked the mechanism below and can see the lever on the gearbox motor moving but nothing is engaging. Also noticed the rear wheels are now free running where as before they were not,is there a simple fix to this problem or do I have to buy a new gearbox? please help as already spent a lot of money doing other things to lawnmower and this is the only problem left now