Setting a constant temp boiler at 65C means that it will hardly condense. The flow needs to be below 53C for full condensation on natural gas. Weather compensation will keep the flow temp below this for about 95% of the heating season, it is far better than on-off control. The boiler will work like the engine on a car with cruise control - it provides exactly the power needed at all times, no more, no less. It does this by continuous modulation of the burner output and pump speed. Most pre-mix burners are at their most efficient at about 30% of full output, weather comp achieves this for you, Alistair.