Bust Flymo Gardenvac

Our Flymo Gardenvac 1500 Plus, has stripped the thread on the mulching head that minces up the leaves that are sucked up, therefore meaning that the fan does not rotate, and now doesn't either suck, blow or shred. I seem to think that its a bit daft having a plastic "nut" on a steel shaft, that will rip out of the plastic, if there is a problem (we don't know what caused it). Wouldn't it be more sensible to have a ratchet system similar to the one on the brushbar of the Dyson Vacuum cleaners?
Any Help on what can be done about the problem, or other advice welcome!

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