invalid utf-8 byte

[Richard Barrington]

Hi there,
I'm building an SAX based XML processor that feeds into FOP, and I'm getting the 
error:
org.apache.fop.apps.FOPException: invalid UTF-8 byte (Check the XML declaration) 
 (code: 0xa0)
HelpXMLProcessorController: Couldn't run FOP Driver;

The code is very simple, eg:
Driver driver = new Driver(new InputSource(new FileInputStream(tempFile)),
                                new FileOutputStream(outputFile));
driver.setRenderer(Driver.RENDER_PDF);
driver.run();
As the error messgae suggests, I've checked the intermediate XML, and the 
declaration is: <?xml version="1.0" encoding="utf-8"?>

Also, Unicode.org suggests that 0xa0 is NBSP, which should be ok, I think.
I must be missing something obvious, so any assistance would be appreciated.
Richard.