Oil fired boiler thermostat?

I'm struggling to get enough heat out of my boiler in this prolonged cold spell. I've been told to turn the boiler stat to maximum because the mains water is now very cold and the stat simply controls the number of degrees by which the water is heated, not the final temperature. Is this true? Is this how a boiler thermostat works? It hasn't made a great deal of difference.

Jules
January 2010