The temperatures also go up if I close my back door, coincidentally to the same temperature I get if I close my case. So it's not the case which has bad airflow, it's my flat.
Okay I have it running stable at 18.5x200 = 3.7GHz. Yay. But it's tenuous. It needs 1.4875V through the CPU, and testing in Prime95 will only work if I restrict it to testing 2 cores at a time. Otherwise it will probably get too hot and die. As it is, it's hovering around 61C with just 2 cores under full load.
The good thing is, nothing other than stress testing tools ever makes the CPU that hot. So it seems it will be stable so long as I keep an eye on the temperatures.
So yay, I finally have a clock speed marginally higher than what I was able to achieve with a £53 core2duo. :-/
Off topic, but are there any apps which will allow me to see the temperature of each core independantly? Core Temp only shows the temperature of core 0.