I've got 4 Mercian frames which are 531; 631 and 731 OS and they all take a 27.2mm seat post. I would take the frame to a decent bike shop and get them to ream out the seat tube to 27.2mm.
Good luck
@Robbiebee started
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I've got 4 Mercian frames which are 531; 631 and 731 OS and they all take a 27.2mm seat post. I would take the frame to a decent bike shop and get them to ream out the seat tube to 27.2mm.
Good luck