Well, HarperCollins have been nice enough to actually answer me about this situation. Here is a portion of what they said...
...Although we do produce our books exclusively in ePub, many of our retailers convert the ePub to other formats before they sell them--as an example, ebooks.com converts our ePub files to Mobipocket and Microsoft Reader. Hopefully you can find an e-book retailer that sells the file format you prefer....
So, now that I have had that "from the horse's mouth" I'm going to be "attacking" the stores I commonly use, e.g. BooksOnBoard.
I appreciate the replies, but don't forget to hassle your retailer/publisher if you want to get anywhere - especially without having to resort to converters etc.
All the best