I tend to use these couple of sites when buying DVDs. Just type in the name and they will automatically compare prices between all major online retailers. These are well worth bookmarking if you regularly buy these things:
Find DVD:
http://www.find-dvd.co.uk/
Billy Bargain:
http://www.billybargain.co.uk/ (Also good for books and CDs)
Amazon are not always the cheapest - you'll find one major retailer such as Play or HMV will drop a price and suddenly the others will follow very quickly. Barry did get lucky with the "24' set - its currently cheapest price is £98.