What kind of flue do we need to install a gas fire on an inside wall - no chimney?

The house is a 60's bungalow with a large ventilated underfloor space with above-ground air bricks on all sides. Can we put a vertical flue up through the loft space, and if so what would it be like? We could create a slim mock chimney if need be. Have thought about flueless fires but am put off by safety issues. We can't use either of the outside walls because they both have huge windows.