they are reluctant to come on when its dull and raining

It may be dull and rainy to you, but there's still a lot of light coming down from the sky and that's what the sensor's looking at. Auto lights are very good at spotting when it's dark, but in daylight hours they often need to be overridden.

One of my "pet hates" is the number of idiots running around in low morning mist with no lights, as they are relying on the automatic sensor to do their thinking for them. The sky can be quite bright in such conditions, even when horizontal visibility is seriously reduced. DRLs have made this worse, as the plod don't seem to bother pulling people over for having no rear lights on.....