Coolant, not water. As it's fairly warm, you might want to dump whatever's in there and run it for a couple of days with a radiator flush. To empty, undo bottom radiator hose. You may have to replace the nasty FIAT hose clip with a new jubilee clip. Otherwise, 50/50 water/antifreeze.
Then bleed -- just fill the expansion tank to max, open up the front bleed nipple, wait till water comes out, tighten up. Then, turn heater to full, try with the rear one (if you have one!). Sometimes the rear one responds better if the engine is ticking over, but, for safety and sanity, try and do it with the engine cold..