I'll start by saying that La Senza are an absolutely amazing company to buy from - excellent service. I have used them three or four times now. The items come wrapped in a gift box, with crepe paper and nice smelly beads scattered in it. The quality of the merchandise is excellent as well.
For this offer, they are doing 5 pairs of briefs or thongs for £10, and postage is £2.95 at the minute. That makes it about £2.60 delivered for each pair of panties - good enough for good quality stuff.
Simply add 5 applicable items to your basket and the discount will appear. Don't forget to go through quidco for a massive 10% cashback as well!!!