500l oil per month is not right - if the tank is not leaking (you would know by the smell if nothing else), then I think you have to look at the way the heating is used.
For context, we are also north with a stone built listed house, oil CH/water, plus an oil aga - we use 2 tanks of oil per year (2400ltrs total). We also have single glazed windows.
Firstly, make sure you have interlined (double of poss) curtains and/or blinds at all the windows and make sure they are closed before it gets dark. Also hang curtains on the inside of external doors.
Secondly, if you already have not, add thermstatic valves to all the radiators, and bleed them as someone has already mentioned in a previous post. I would also be tempted to ask a plumber or CH engineer to come and balance the heating system. Plus check what temp the boiler is set at.
Check where your room stat is located as this will regulate the temp of the house and how much time the boiler comes on. If it is in a room which is warmer, possibly one where you use the open fire, it will mean the remainder of the house is cold. If you do not have a room stat or stats, get them - you can use wifi ones if need be.
Lastly, the way you need to heat old, stone houses is different to newer ones - it takes a while for them to become warm, so just raising the stat to 16 and allowing the boiler to fire for an hour is not going to work. The heating needs to be on for longer at lower temp - never allow the house to get very cold as you will just waste oil trying to raise its temp again.
Water does not need to be heated for as long as CH - so you may be able to reduce the boiler being on for the water.