I think I have found the cause of the problem in my case. At some point Media Center had tried to download TV/guide information and failed because the Internet link was down.
This caused the "download of the latest setup information for your region" to fail when I did a new TV setup after clearing the TV database. I then tried a manual dowload from the MC menu, which also failed and told me it could not be done because of previous failed download attempts.
I think this stops 7MC from loading the EPG data from the DVBlink server, and possibly the channel sync as well.
Yesterday I added a completely new client to the network and it worked without a hitch. From which I conclude that this is a client side problem and that just deleting the TV database files from the programdata\microsoft\ehome directory won't fix it. MediaCenter needs to be completely reset to a post-Windows-install state. Not sure how to do that so I will wipe the offending client and re-image it from the fresh client I installed yesterday.
Hope this helps
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