You say you have an ‘old’ oil boiler but didn’t say what other controls on the system.
Some systems have a frost stat in the utility room or similar that is set to 4/5 deg C so of the temp in that room drops to the set temp, the boiler is forced to fire, regardless of your timer settings. This keeps the house between 4-9 deg C depending on the hysteresis of the AT&T ( I like that word,).
If you have a room stat that controls the boiler, set it to whatever you feel comfortable with – if the are pipes in the loft, you might want to keep the house warmer than if not, to alow the loft to stay above freezing. If no pipes in the loft, I’d be happy with 4-5 deg c.
If you don’t have a room stat, and use the boilers thermostat to vary the temperature in the house then turn it to its lowest setting and leave on 24/7.
HTH