Cheapest 4GB DDR3 kit on the web. Next cheapest is about 20% extra and will also be no-frills RAM.
EXTRA VALUE 4GB (2x2GB) DDR3 1333MHz Memory Kit 1.5V CL9
"I decided to save a bit of money and go for this over more expensive branded memory, and I'm very glad I did. It works perfectly, passes all tests, and was correctly recognised by the motherboard as 1333MHz."
Price doesn't include delivery (£1.49), but you can get free delivery by spending £49.99+.