As for the spelling "Widecombe" or "Widdecombe": both have been used in the past. The first is used these days, but the song title is usually spelt "Widdecombe Fair" - remember that there was no standard spelling for place names in the UK until Victorian times (the standardisation of place names was a result of the advent of the railways, and the need to produce signs and timetables). I'm guessing here, but the old song probably predates this, and if so the spelling would not have been incorrect at the time it originated. Steve Knightley just used the same spelling as the old song title, whether or not he is/was aware of the modern "official" spelling.