Hi Andy,
What kind of Avant do you have, type nr?
The problem is that the avant automatically detects videosignal on the Camcord input.
You can try :
1. Use the decoderscart socket om the back, this supports also CVBS and you can probably give it name like VAux, Vtape2. This depends on the type of Avant.
If you can't give it a name, then probably you can use TV 99 and tune that channel to the decodersocket (in the more option at tuningmenu)
After pressing TV 99 it searches for videosignal at the decoderinput.
2. Maybe you can rename the Camcorder to VAux in the setup menu.
Go to the menu, setup, and go to "Menu" line and press menu while you're standing on it.
It depends on the type of software, but you can rename Camcord to Vaux. Perhaps it stops responding automatically to the videosignal of the CCTV. I never tried it, but maybe it helps.