
Hi Simon,
Here's a variation on Wayne's suggestion that uses fewer formulas:
Percent is the fraction of the whole sample. The first formula, in E2, calculates the fraction (the same as shown in E6 of Wayne's table. Choosing Percent as the format displays the fraction as a percent.
E2: =COUNTIF(B,"Y")/COUNTA(B)
Since each respondent must choose one of two answers, the fraction answering "No" must make up the rest of the whole sample. The second formula, in E3, subtracts the "YES" value, in E2, from the whole sample ( 1 ), to obtain the fraction answering "No". Formatting displays that as a percent.
E3: =1E2
Regards,
Barry 
