The o d is 15, the i d is 13mm or a bit less. Pex pipe (as at screwfix) has a slightly thinner wall than polybutylene (hepworth/polypipe). One of them is 1.2mm wall thickness, can't remember which.
I know this is an old thread, but as it's at the top of the Google results for others (like me) who aren't looking for PEX to "do the job it was made for", I thought I'd update this thread with the I/D information.
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