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gillies-
I think what had happened with mine was the topcap was firmly tightened onto the expander, which itself wasn't done up quite tight enough to grip the steerer, but just enough to not be able to remove it. I vaugely remember having to tighten and loosen the expander bolt quickly whilst holding the top cap still.
I did replace it with the slightly beefier version, but have since gone back to the super light version. There wasn't anything wrong with it, I just didn't want the hassle again. However weight weenyism is a strong thing so I put it back on
TooTallTim
Can you somehow hold the topcap still whilst using an Allen key to tighten/loosen the expander? Something similar happened with mine and it took what I can only describe as 'serious fettling' to get it out. All was fine in the end!