For those of you who use Castrol Edge engine oil, Homebase are selling the Edge 5w30 for £7.98 per litre, and the Edge 0w30 for £6.98 a litre. This is the cheapest i have ever seen this oil.
You can expect to see it for sale from anything upto £18 per litre, so grab a bargain, it's bloody good stuff.
If your'e unsure what oil is suitable for your car go to Castrol.co.uk and then click on the "what oil is recommended for my car", link, put in your reg and it will tell you.