Mine turns about an inch with or without key..... 
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I have one on mine but lost the key so cant check ! On the Disco 3 I had it always took a bit of force and wiggling of the key to remove the Towbar there was never any logic to it . I suggest you put it back on the car and try a bit of jiggling the key and turning the knob . Hope that helps.

Last edited by googsy2 on 16th Jun 2011 5:43pm. Edited 1 time in total

Mine twists a bit with or without key but will not pull out more than a few mills so the hook will not drop out  
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I had two keys and was using wrong one....

Key turns 180 degrees then knob pulls out easily and turns clockwise and the hook just falls out.

I then removed armature after buying large enough sockets to do the job!

Finally sprayed up the underparts in Black Silk and it all looks like new  
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