An "RJ45 cat5 e crossover cable". About 5 quid in maplins. If you are really wanting to pinch the pennies I found my laptop and my sisters worked with an rj45 straight through cable we got for £1 in poundsworth.
However the top will be the one that will definately work. I think that mine and my sisters laptops were compatable due to the software installed.
Just go to maplins and ask for the "..." above they will get what you want.