Thanks for the reply ATDrake, that's exactly what I was after.
I'm using Sigil (which doesn't seem to allow you to set a start page) and when I unzip my ePub files and edit the OPF there's no start item in the guide.
I added one manually and reinserted it into the ePub and it worked fine. I was using the Kindle Previewer to convert to MOBI and I guess it was adding the start tag to one of the pages using some kind of internal best guess method when it does the conversion.
Do you know if there's an easier way to edit the OPF data for an ePub file without unzipping it?