Further to above, I have just noticed that attempting to connect via the Check for updates method no longer creates any logs in my firewall - which I am pretty sure it did before (when I was getting the error message).
In case it is useful, I tried again whilst running the log function - extract of what appear to be the relevant lines below:
0006203: 5788: Reader: CInternetReader::Open: Start
0006203: 5788: Reader: CInternetReader::Open: Opening
http://www.musicex.com/cgi-bin/redirects/MC/v12/redirect.pl?type=Update&prodname=Media Center&version=12.0.365
0006203: 5776: Reader: CInternetReader::Thread: Start
0006203: 5776: Reader: CInternetReader::DownloadFromHTTPURL: Start
0006312: 5776: Reader: CInternetReader::DownloadFromHTTPURL: HttpSendRequest error: 12029
0006312: 5776: Reader: CInternetReader::DownloadFromHTTPURL: Failed (header status code: -1)
0006312: 5776: Reader: CInternetReader::DownloadFromHTTPURL: Finish (109 ms)
0006312: 5776: Reader: CInternetReader::Thread: Finish (109 ms)
0006312: 5788: Reader: CInternetReader::Open: ConnectToURL failed, m_bConnectError=TRUE, URL=http://www.musicex.com/cgi-bin/redirects/MC/v12/redirect.pl?type=Update&prodname=Media Center&version=12.0.365
0006312: 5788: Reader: CInternetReader::Open: Finish (109 ms)