you may not be able to get the DT to stay on the frequency you input, and it really does not matter...
anytime you press the Guide, Service, Interactive or Box offixc buttons on the Sky remote, the box will automatically loook for frequency 11778. this is because on this frequency sites the EPG backgound music, which plays ( or tries to play) whenever you access the menus).
Also once you have a picture on your screen it makes no difference what default transponder frequency you have, as you box will look at the relevant frequency for the channel you are watching and will override whatever default frequency you have input - so if you are watching sky news the box looks at frequency 12207, for bbc news it will use 11954., if you go to eurosport2 it will use eurosports 2 frequency on the north beam!
the box will override the default transponder freqquency you input and use one of the other 90 or so frequencies it needs to for the channel you want to watch!