Problem with Feeds

My blog is at http://lostaddress.org and my feed is at http://lostaddress.org/feed and I am using the latest version of the Fluid-Blue theme.

For a few weeks now, my feeds have been down. I checked them with FeedBurner and was given the following error:
Error on line 2: The processing instruction target matching “[xX][mM][lL]” is not allowed.

When I follow the link ( http://feedvalidator.org/check.cgi?url=http%3A%2F%2Fwww.lostaddress.org%2Ffeed%2F ), the error page gives me:

This feed does not validate.
* line 1, column 1: Blank line before XML declaration [help]
Source: http://www.lostaddress.org/feed/

1.
   2. <?xml version="1.0" encoding="UTF-8"?>

Having Googled and searched here, I am aware that there must be a blank line in a file or files somewhere. I have been through wp-rss2.php and wp-atom.php and wp-config.php and functions.php to look for blank lines at either the start or end of the files and found nothing. I have also disabled and re-enabled plugins to see if there is any change. I have found nothing.

Can anyone suggest anything else I could try? I suspect my next step may have to be that I change to the default theme to Kubrick for a while to see if anything changes.