I'm probably stating the obvious, but are you setting your television to output 3D? If you look at the BBC HD preview, the limited 3D content is shown in split screen as you describe, if viewed on a 2D-only display.
You don't mention which TV you have, which might help if you more specific inquiries.
2. Switch your Sky+HD box on and make sure the box is connected via HDMI cable.
3. Using your TV remote control, switch the source to the correct HDMI channel. The AV key is located at the top right of Panasonic remote control. Press AV until the Sky picture displays.
4. Select Sky 3D channel. (Sky Channel 217)
If the on screen message 'Call to Upgrade' displays, check that you have a current subscription to Sky World with the HD pack.
5. Side by side image displays until your 3D-ready TV is switched into 3D mode.
6. Switch your Panasonic 3D-ready TV into 3D mode by pressing Viera Tools Key on TV remote control.
7. Use the directional navigational keys to select ‘Side by Side’ mode.
8. Press OK button to select.
9. Switch your Panasonic 3D Glasses on and put them on, making sure the batteries are inserted correctly and in operation.
10. You should now be able to see Sky 3D.
Might be stating the obvious but have you switched on the 3D glasses? Are they charged?
When I turn to 217 a message appears onscreen telling me a 3D image has been detected - I then need to press the 3D button on the remote (at the top beside the power button). The image jumps to an out of focus image until I put the glasses on.
i have discovered the problem. The 3D TV was in the kitchen, running off a magic eye. When i moved it to the loung and plugged the HDMI cable directly into it from the sky box, hey presto 3D works! Didn't know it had to be connected directly.
I guess my only solution now is to run a HDMI splitter into the other TV or get another sky box or a CAT 5/6 cable.