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I used a screwdriver to bend it out to the same angle and the 26.8 fit well!
Yep, this isn't all that uncommon; the giveaway is if the edges of the slot aren't parallel. It's a good idea to tickle the inside of the slot with a file after levering it open again, to ensure you haven't created a slight dag that will scratch the post.
Kimmo
So on closer inspection it appeared one side of the seat collar(?) where the bolt goes through was at a different angle to the other. I used a screwdriver to bend it out to the same angle and the 26.8 fit well! Clamps down solidly too.