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.
We still need to know your post code in full so we can determine which os the possible transmitters you might be using and whether there are any known problems. Without that information we cannot help.
According to the data contained in the boxes below yours with the post code (thanks for that) you should be using the Mendiptransmitter which is at a bearing of 182 degrees from your location. That is a full service transmitter using channels 49, 54, 58, 48, 56, 52, 33, 35 and 51. Channels 58, 33 and 345 are HD transmissions and you need a full HD set, not just an 'HD ready' type, to get those services. Channel 51 is used for Local Bristol services.
So check the approximate direction of your aerial and then check each of these for signal strength, but do not perform a retune yet. You want between 65% and 85% strength, any less or any more will cause apparent 'loss of signal' symptoms and picture and/or sound problems.
Let us know what strengths you get, then we may be able to advise further.
If your aerial is pointing approximately North then it is not pointing at any of the available transmitters and may be getting some signal;s from Kings Weston Hill, which will give very poor reception at your location.
If it is indeed looking northwards, then it needs to be changed to look at Mendip. However, when tuning be careful to avoid any signals from the Ilchester Crescent one.
Our posts overlapped. I susopect that some of the signals you are trying to watch may be from Ilchester Crescent, so giving poor results. Manually tune to the given channels from Mendip and let us know if that has improved things.