Formatting for uploading to Ibookstore

[NichePublisher]

Hi everyone. I have a large selection of backlist titles which I am in the process of converting into ebook format. Kindle is easy - I generate an html file and then use mobi pocket creator to make a prc file. It uploads without any problem onto Amazon.

Problem is I cannot find anyway to generate an epub file (either from the prc or original html) which will be acceptable to Apple. Calibre creates epubs which are fine on readers I use on the PC but the Ibookstore rejects them. I know there are companies that will sort this out but I'd rather do it myself.

Any suggestions will be much appreciated. Thx