Having tried to remove the rear wheel by hitting the wheel too hard!...The axle has come away from the transaxle. I know I've learnt a lesson the hard way but mistakes happen and I want to fix my mower tractor!
The axle half-shaft is retained internally by a circlip. You will have to remove and strip the axle to repair - and hope that the axle casing is not too badly damaged to retain the new clip.
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Try fixfault.com to fix lawn mowers.