Hi quinny 1982,
The first thing to check do is look in the manual on how to do a factory reset.
Once you've done that look at this
page on how to check the signal strength the TV is receiving. Anything less than 50% and most tuners will not display a picture.
If the signal strength is okay then try another auto tune.
If your still having a problem then I'd suggest you you might have a faulty unit. However, a lot the time companies like to blame your reception area, TV Antenna (Ariel) or cable length or quality. Yes, old coaxial cable used for analogue TV is not very good when used with digital equipment (not a surprise)!
From what you've described, if you can only get a mono signal I'd be surprised it that's actually a digital channel.
Don't forget to check your Antenna connectors.
Hope this is helpful.
Best of luck.
Spare head 2.