No, you definitely don't need an accent. It's a long vowel, because an unmarked (i.e no accent) stressed vowel is always long before b, ch, d, dd, g, f, ff, th, e.g. mab /m??b/ (son), hoff /ho?f/ (favourite), peth /pe??/ (thing). So you don't need a circumflex to tell you it's long, because it always is before an f.
With a name like 'Siân', you need the circumflex because before an 'n', vowels can be short or long, depending on the word. In this word, it's long - the circumflex tells you that.