The most efficient is to set the boiler thermostat at the lowest setting
consistent with also heating the house adequetely.

I forgot to say, that doing so also has a safety benefit. Traditional
radiators had a peak temperature of 82C. Using the method above, in normal
weather so that the dial can be right down, you may find 60C or even 50C to
be acceptable, which is far less likely to scald a child.

Christian.