We are here to help, though the real experts are elsewhere at the moment.
It would help us if we knew which switch-over area you are in. I am in Yorkshire so affected by Emley Moor/Sheffield/Chesterfield switch-over.
Do you have an aerial amplifier powered by the Humax?
Is the Humax connected directly to the aerial?
Can the TV connected to the Humax detect any channels (if it is a digital television)?
More questions and help now:
Have you tried a Default Reset prior to the re-scan; if not try that. (Note the Password resets to 0000.)
Next. Select Manual Search from the Installation Menu and step through the channels. At the bottom of the screen you will see the Signal Strength and Quality Indicators; they will tell if you are receiving a signal and how good it is. Most people appear to need greater than 50% signal strength to generate a 100% quality signal. (Note no-one knows what these figures really mean in scientific terms but above is a working guide.)
If you have no good signals then you need to go through the Default Reset process above with the aerial disconnected (switch off the mains power at the back first) so that the Humax "forgets" all its tuning information and starts again. The Humax will report no channels found but after that, switch off the power again (at the back/on the wall), re-connected the aerial and carry out an installation search again.
Finally, if you do find some 800 channels in the guide you may need to do a manual search.