There are many other possible sources of air leaks but here are some with some possible solutions
Open chimneys draw a lot of heat out of a room as well allong noise, dust and animals access into the room.. Various DIY methods of blocking chimneys are effective (e.g. a fixing a board to fireplace). Installing a damper on top of the chimney is also an option.
A simple propriety option is to use a Chimney Baloon which is an inflatable bag that is placed into the mouth of the chimney. This is a simple solution with many benefits including that if a fire is accidentally lit then the bag simply disintegrates and burns up. These can be purchased online from the manufacturer at http://www.chimneyballoons.com.au/
Ducted floor vents
If the register does not seal then replace with an adjustable register that seals the duct when not required.
Register deflectors are useful to deflect into the room instead of rising directly across cold windows, as many registers are positioned under windows. Adjustable floor registers are available from Bunnings and other heating specialists.
Adjustable floor registers are available from Bunnings and other heating specialists.