It seems that you are having some issues with your Slingbox PRO ever since you moved the TV air antenna, which has caused the "weak or not video signal detected" message.
The are several causes for you to get this message. The following link will provide you with the steps to follow in order to fix this issue.
Thanks for that. I'm now pretty certain that I need to retune the Slingbox to home in on the signal from the transmitting mast that my antenna is now pointing to. Do I need to reset the whole thing and do a "set up" from factory settings or can I just do a retune from where I am now and if so, how do I do this? The Sling box worked fine till I redirected the antenna to another transmitting mast which gives a better signal for my telly.