Windows Update error 80200011

Running Vista Home Premium (32 bit).

Previously, using Ethernet/ADSL or Wi-Fi to connect to the internet I
have had no problems with Windows Update, however since switching to
dial-up via an HSDPA USB modem (Huawei E220) to connect to mobile
broadband, Windows Update fails with error 80200011.

I can download files via IE, including updates from the MS downdload
site, and can check for and see pending updates but cannot download
them. I have traced the fault so far to the BITS client, and the
error is listed here:
0x80200011 BG_E_MISSING_FILE_SIZE When BITS sends a HEAD request and
the server or proxy does not return a Content-Length header in the
response, BITS puts the job in ERROR state with
BG_E_MISSING_FILE_SIZE. Check the proxy and server to ensure that they
are configured correctly. Some versions of the Apache 2.0 proxy server
are known to exhibit this behavior.

When looking at the BITS job that WU creates, I can see the URL and
can then copy this to IE and successfully download the file, so it
appears to be an issue with how BITS downloads files, as opposed to
how IE downloads. As far as I can tell there is no proxy, so the
Apache reference may be a red herring. Also friends using the same
modem/service, but with Windows XP have no problems.

I have tried clearing the SoftwareDistribution folder, reinstalling
WU, checking BITS (no errors) etc. I am also running NIS2008 but
disabling this does not help. The error is also present when I try to
use the Windows Update Catalog or Windows Defender (as I believe both
also use BITS).

Any suggestions or advice gratefully received.