After looking at it more closely and reading BrowserUK's message it appears that Peek does behave as you suggested. I eventually fixed it by using a blocking read instead. I.e.

      int totalBytes = 0;
      int count;
      char[] buffer = new char[4096];
      while ( (count = process.StandardOutput.ReadBlock(buffer, 0, buf
+fer.Length)) > 0 )
      {
        totalBytes += count;
      }
[download]

Sorry for it not being ultimately a Perl question but thanks in any case :)