Kenwood Chef mixer - PM900

My friend has just given me one of these, but it will not open more than a few cm. The hinge/counterbalance seems to be misaligned or possibly twisted - meaning that the top and bottom of the machine is slightly out of alignment and so it collides at the heel preventing opening.
Can anyone tell me how I get the two halves of the mixer apart? There is a small hole to the rear of the mixer where you can view the end of the hinge pin...but I'm not sure what to do with it. Do you press/tap it in to release?
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