Well, since I have one to hand...
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Prise off the cap from underneath, working round the back until it comes free.
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Unbolt from the seat.
View attachment 121490Use the height adjustment roller with your thumb to release the adjustment rod. Keep unwinding until the rod drops out.
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Remove the roller and unscrew the torx screw
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The armrest cushion slides forward and up
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To remove the cover, or refit as here, carefully lever each slit over the respective arrow. The cover slits are not hemmed, so don't pull hard as the fabric will rip.
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Reverse steps to fit back on the seat