This was posted over a month ago, but i believe they've brought the offer back.
Free Samples come in sizes:
S/M 17 - 19kgs (38 - 65 lbs) Boy or Girl
L/XL 26 - 39Kgs (58 - 85lbs) Boy or Girl
Also get a coupon worth £5.19 towards a purchase of Underjams with your sample.