I think that a fundamental mistake that people make hiking in the rain is that when it starts raining they just put their rain gear on top of the existing layers. Since most of the time the temperature remains about the same that causes unnecessary sweating (IE if you were warm enough before you will get much warmer with a more or less vapour barrier layer on top) , it also means that when taking a break the wet layer underneath will cause the body to chill very quickly. IMHO it is better to take one layer off, put the rain gear on, and add that extra layer back on during breaks.
Tom, usually a light wool top is my only layer under the rain jacket, but growing up in Northern Italy those 5 kg really thick woolen jumpers (all white in NZ) were the ones we wore when it started to rain.
Franco