Henry, you are totally vindicated. Thank you so much. Now that I have my Kindle Paperwhite, I have been able to check your recommendations and they are spot-on. Changing the pxs to ems in Sigil does the trick. There is no longer any need to go via Word or even involve Calibre at all. Having done the necessary in Sigil, I uploaded my unfinished book as draft to KDP and downloaded the conversion back again. It was perfect.
Case closed. Thanks again.
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