oil fired heating

What is the most economical way of using oil fired heating.

a) Turn on heating for 1 hour at a time when it gets chilly

or

b) Turn down thermostat on boiler and leave heating on full time

The manufactures test them based on running full tilt. So you would assume that the boiler coming on to give a big blast every couple of hours would be the most economical. But for comfort levels in your rooms that doesn’t work save for the best insulated houses which will hold the heat.

Fit thermostatic valves to the rads so that you can have different temperatures in each room.

Don't turn down the boiler thermostat, you'll run the risk of condensation in the boiler, followed by rust then failure.
Thermostatic rad valves as demooivre suggests are well worth the effort.
A room thermostat controlling the boiler may also be an option.