You need to enter a hidden menu and then you will be able to set an initial input.
To get into the hidden menu, go to the setup menu and highlight the language option and then press and hold (for about 3 secs)the surround sound button on the remote - you will then get a screen with some new options...just find the one for initial input and set to what you need...
Embees, you should modify your setup. There is absolutely no reason to use the panasonic tuner unless you have to have analogue TV. You might be able to sell it on Ebay, and get yourself properly sorted.
No, what else do you want to display on the screen? It's perfectly possible to do without, but I can't tell you what you need to do without knowing this. You can use the VGA port, or you might havr to find a video card for a 5 series. There must be some on somebody's shelf and you are in the right place to ask. If all your sources are RGB like the cable STB, you won't need a card at all.
If you can get them all to do RGB on scart, then you could use a scart switch box and a JS RGB to VGA converter, and disconnect the tuner. Cost just under £200 I should think. You might be able to sell the tuner for that, it works with the 6 series, get a better picture and more control for no money. Are we good to you?