They should normally do this for you but it's really simple to do. If you have any difficulty with the following steps, if you go to help and info from the home menu and select help videos, there is a video on how to set up HD.
1. With the scart lead still connected, attach the HDMI cable to the box and TV
2. Press Home, go into settings and select 'Change Display and Audio Settings'
3. On TV Display format, use the right arrow to change to 'HiDefHDMI'
4. On V+ Box HiDef output, choose 1080wide if you want 4:3 pictures to stretch to the screen size, or 1080 if not
5. Press OK and select continue
6. Switch over to your TVs HD Channel and if you can see a picture, press text to confirm.
Ben