Heating old house

Just purchased old Georgian house, 4700 feet sq over 3 floors. Currently heated by oil-fired Aga.
Any suggestions as to alternative and cost-effective heating system? Someone has told me to look at a wood gasifier as there is substantial forest with the property.

Jealous I am in some ways of you. Is it a listed house?

I assume a wood gasifier requires constant looking after. So the question is do you have the time/energy or staff to feed and maintain it. My grandparents lived in a big historic house and it was probably one of the first in the country with central heating. Big heating pipes throughout with cast iron radiators. This all lead into a boiler house with a very rusty boiler which looks like like the boiler from a small steam engine. It had probably not been used in over 100 years. From the days when the gentry had multiple staff to look after such a system. The house was always cold and damp with one room heated by a gas fire.

In numerous threads here it appears the question you need ask is 'What can I do to stop heat leaving the house'. I suggest you start there and concentrate on lagging rather than heating.