I would go for 250W and I recommend having a dry run first. If you need it you'll probably need it in a hurry so figure out first where you're going to hang it, how you're going to keep the ewe away and take a thermometer and work out how high it needs to be, bearing in mind that it's the temperature at the height of the lamb that counts, whether lying down or standing up, and if it's in a poor way it won't be able to move away if the lamp's too hot. We always pop a jacket (rectangle of old sweatshirt or similar, with crosses cut at the front where the legs go through) onto small lambs or those born on a very cold night. It just seems to give them that extra bit of protection and gets taken off by day 3. I know the "our ewes lamb on the hill with no intervention" boys would think us soft in the head but our scanning to weaning losses are generally 2% or less, not 20%!