Qn. Have the squares on your handles rounded off and the locking bolts will not tighten. If so then replacing with these handles will solve your problem.
GTech CLM021 Lawnmower - Grass box flap
GTech CLM021 Lawnmower - Top Handle with Fixings
GTech CLM021 Lawnmower Lower Handles 465 mm length - USED
Gtech Cordless Lawnmower CLM021 - starting bar
GTech Cordless Lawnmower CLM021 - Front Axle