You're absolutely right - how the font displays in ADE is correct. I checked it in Adobe Indesign. Sigil and Chrome both display it incorrectly
That's fine - I'll just use regular then. It's a real pain in the ass not having actual devices to check these things on because the software's a nightmare.
Thanks again for your help. Just out of interest - if you made a test file, did it display as the same weight in iBooks?