According to Aquascutum's women's size conversion, size 40 is US Large. The fit is relaxed, somewhat oversized, so it MAY fit XL as well. Go by the measurements for your best fit, and by how you like your sweater to fit.
Particulars:
Size: 40 (US LARGE, relaxed fit).
shoulder to shoulder: 16 and 1/4"
bust: 41" circumference
sleeve length: 23 and 1/2" (end of sleeve to shoulder seam).
length of sweater: 23 and 1/4"
center of bottom band, side to side, across, unstretched: 16 and 3/4"
weight before packaging: 7.3 oz.
fabric: cashmere silk blend.