[PROBLEM] Can only see video /main display in top left corner of my TV

[bkf11]

Hi,
I'm new to Xbian and to RPi. Have just installed Xbian onto my SD card in a Model B RPi and plugged the HDMI port of our Panasonic TV and also into a small 7" HD monitor (from Adafruit) and in both cases the main display and the video only cover perhaps 2/3 of the screen at the top left corner. The mouse pointer can go across the entire screen. See attached photo. I've tried to muck around with video settings and calibration but that just changes things slightly. It's very odd. Any help appreciated.

Configuration:
RPi model B (recent model with 512MB RAM), EMTEC class 4, 8gb SD card, powered off a 2.1A port of a large USB power bank (temporarily), with USB mouse and a USB SD card reader plugged into the USB. Fresh install of Xbian 1.0a5

I tried Raspbian yesterday and that worked OK but I was losing the edges of the display (need to change overscan setting?) and then I think I deleted the OS by trying to make the rest of the SD card visible and formatted for Windows. So tried Xbian instead.
Thanks, Benjamin