After the analogue switch off, digital transmissions increased in power by a factor of 20. Your box is receiving distant transmitters on lower frequencies and allocating them to the 'slots' for BBC1, BBC2, ITV etc.
The channels you actually want will be in the the 800 number group. Use them until you have the opportunity for a manual retune.
The individual channels are grouped together in multiplexes (also called muxes). To do a manual retune you need the channel numbers (or possibly frequencies) of the multiplexes from your local transmitter. Look them up on line. Note, this isn't the same as the channel numbers of the stations.
On your set select menu->setup (or whatever the manufacturer has chosen to call the menu section where you can do a manual retune). Select the channel numbers to tune. Save the results. You might need to delete the earlier tuning with an automatic retune with the aerial unplugged (but most likely not, with a recent set).
Now delete all the junk channels.
Simples.