To cut a long story very short, I've always had a problem in accessing Windows Update from behind my Linksys BEFVP41 VPN router (which otherwise is excellent); well, I had to reinstall my main PC over the past couple of days (hence the lack of posts) and this became a major problem...so after some fiddling with settings, I discovered that changing the MTU to 1454 totally solves the problem. This fix also means I can put my external servers on the same segment as my dev machines again - giving the 100mbps access to each other again as this problem also caused problems for using externally facing web servers behind the router - yay!