If there's a 'replace channels' option, rather than 'add' or 'edit' them, it's 'replace' that you need to use. (That wipes everything from the list and starts again).
If that doesn't work then, as the problem is clearly related to low signal strength, you need to check your aerial. (i.e. make sure that the wind hasn't knocked it out of position and that everything is plugged in properly). If you can get most channels OK a simple signal booster (from almost any large supermarket) should fix the problem. Otherwise you'll need to call in an aerial fitter.
All of the above assumes that the problem hasn't come about simply because your local transmitter is operating on reduced power (as is often necessary to enable engineering work to take place). Check here:
http://www.ukfree.tv/maps/live