Lawn mower drive problem?

We have one of those four wheeled four stroke mowers, with the rear wheels powered - except the drive to the wheels has almost declined to nothing. It can turn the wheels when they are clear of the ground, but not under load.
The drive is taken off engine, just above the rotary cutting blade, via a small V belt. The belt is successfully driving the little gearbox on the rear wheels and the hand operated 'drive lever' is operating the arm on the gearbox. I have also tried taking up slack in the cable which operates the arm. So the problem is obviously in the gearbox thingummy.
I assume there is some sort of clutch inside the 'gearbox' and it is this which is slipping(?). Before I attempt it, has anyone pulled one of these apart and sucessfully repaired one?
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Regards,
Harry (M1BYT) (L)
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My mower has a sort of belleville washer clutch (curved face washer) I had to shim it up to reduce the play to get it to work better. Yours MIGHT be like this.
mrcheerful
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mrcheerful . laid this down on his screen :

Thanks for that, I didn't want to try removing the gearbox cover to find bits flying out all over the place.
--

Regards,
Harry (M1BYT) (L)
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