Wow, we don't get many visitors from down there
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You shouldn't need to retune your TV - it's probably better that you don't try. Point your aerial at the transmitter in the south-east of Truro. See this map - zoom in if necessary
http://www.ukfree.tv/transmitters/tv/TRURO
But if you live in a dip, you may be out of luck, and will need to get a roof aerial installed (or maybe even satellite dish). The postcode checker here will tell you if you can expect to get Freeview
http://www.freeview.co.uk/availability