To deal with the problem you must clear the channel list completely and then rescan - if your box has it in the menus, please the 'installation menu' to do an initial scan or a reset to factory settings or First Time Installation. You MUST delete the entire existing list of channels. On most boxes this technique can be also be used:
2) unplug the aerial from the Freeview box by disconnecting the cable from the 'RF in' socket;
3) wait 30 seconds;
4) plug Freeview box (or idTV) mains back in;
5) do a complete scan for channels - it will fail without the aerial. (This may be in the installation or initialization menu, and is distinct from any 'add channels option'). Once this is done your channel line up should be empty;
6) reinsert aerial by reconnecting to the 'RF in' connection;
7) do a complete scan for channels again.
The Freeview channel line-up provided by six "multiplexes" - each of which carry five or more TV channels, radio channels, text services and EPG data. In this diagram each ROW represents a multiplex. If you are still missing a whole multiplex (ie everything on the row) you may need to replace your aerial with a wideband type, purchase a larger aerial or you may have interference from a VCR, games console, Sky Digibox or similar.
Please do not retune your equipment, you will be likely to lose what channels you can get!
The problem is probab;y due to atmospheric conditions that are common when high pressure is present and entirely natural. There is nothing that can be done about the effect and it is not a new phenomenon, so 'global warming' has no bearing on it either.
Just be patient as the predictions are that it may well cease to be a problem from around next Thursday.
Further to my previous post, I have spoken to 2 other households in my street who are reporting the same problem.
We all have our own aerials, so we can only assume there is some kind of transmitter problem, most likely due to the high winds.