Did a bit more reading up and found out it needs to be 0.3-0.5 ohms resistance and at least 100 watts power rating.
I've ordered one of these from RS. Part Number 188-065,100W R47, which is 0.47 ohms. It's an aluminium clad module with a lot of meat on it. It's surprisingly hefty and should last the lifetime of the car.
http://uk.rs-online.com/web/search/s...duct&R=0188065