We employ people on a seasonal basis, going to well over 200 for three months of the year, but otherwise only two or three. Are we caught by the legislation?

To calculate how many employees you have, you need to use the average figure over 12 completed calendar months (or however many months you have been operating, if shorter).

In your case, you employ 200-plus people for 3 months in the year (= 200 x 3 = 600), plus 3 employees for 9 months (= 3 x 9 = 27), making 627, to be divided by 12 to reach an average monthly total of just over 52. You therefore fall within the ambit of the legislation.