Sendit and a few other retailers are selling all sets (1-6) of Poirot for £4.89 each. I have previously seen them go down to £5 but it's been with some of the sets not all. Tesco recently had them all at £4.99 but they've gone back up to £22.99 now. With 4% Quidco you get another 19p off.
Play.com has the new collection (Set 7) which is due for release on 06/10/2008 for £16.99 but that will likely go down later on.