I thought I'd update as it might be useful to someone in the future who encounters a similar scenario. After leaving it sitting around for a few days with the bolt open I gave the inside a quick clean with wire wool. 27.2 now fits neatly. So I assume it just had the wrong size post fitted previously. Thanks to all for the advice.
I thought I'd update as it might be useful to someone in the future who encounters a similar scenario. After leaving it sitting around for a few days with the bolt open I gave the inside a quick clean with wire wool. 27.2 now fits neatly. So I assume it just had the wrong size post fitted previously. Thanks to all for the advice.