The wording of the answer suggests that the setter of this question believes Dave Swarbrick's instrument was really a fiddle, but that 'violin' should be allowed as an alternative answer.
As someone who has played a violin for over fifty years, and predominantly in a folk environment (Dave Swarbrick was my hero in this field), I've always regarded the belief that the violin and the fiddle are two different instruments as an affectation.
Wikipedia notes that the term 'fiddle' is "used by players in all genres including classical music". It suggests that a fiddle can be any stringed instrument that's played with a bow, but (somewhat contradictorily) that fiddles may be set up with minor alterations such as a differently shaped bridge. To which I say "nonsense – it's still a violin".
Wikipedia goes on to explain that "fiddling tends to produce rhythms that focus on dancing, with associated quick note changes, whereas classical music tends to contain more vibrato and sustained notes. Fiddling is also open to improvisation and embellishment with ornamentation at the player's discretion – in contrast to orchestral performances, which adhere to the composer's notes to reproduce a work faithfully.
I wouldn't disagree with the suggestion that there are many ways of playing the violin, and Wikipedia is describing the two extremes. I'm quite prepared to believe that the term 'fiddling' grew up to describe the less formal way of playing. But if that is the case, I would argue that the two terms for the instrument ('violin' and 'fiddle') later became merged, and ended up being effectively synonymous. The violin and the fiddle are one and the same thing.
And finally ... I cannot finish this piece without mentioning that Dave Swarbrick was the only person I know who could play the violin, sing, and smoke a fag at the same time.
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