Yes, as above, first job is to bleed the radiatore.
There should be a bleed screw on the radiatore (inside the engine bay) by the top corner, nearest the headlight. Just undo it slowly (cold engine) until air or coolant comes out. Don't unscrew it all the way, otherwise coolant will come gushing out and you'll have a job getting the the screw back in whilst coolant is pissing out over your feet.
Once coolant comes out, tighten the screw back up and top up the coolant to "MAX" on the header tank. Then bleed the heater. Turn the heater to MAX heat (you don't need the fan on, as some people say) to open the heater valve and then with the engine running, unscrew the bleed screw by the bulkhead until coolant comes out.
Be careful not to trap anything in the moving parts of the motor as you lean over it. Then tighten the screw again.
Drive the beast around for a while until it gets fully warm...The radiator and the heater should feel hot now.... then let it cool down.
Repeat the bleeding of the radiator only. If there's air in there, repeat the bleed/top-up and then drive it around again. Really you want to do it until there's no more air in the radiatore and no more air comes back. Then job's a good 'un.
If you keep getting air in the rad' then it's bad news.. you have a head gasket failure and nothing anyone else thinks or says is going to change that..
Now, the gas in the rad' had to come from somewhere. It might be a result of your missus boiling the coolant.... and that's not her fault since the car shouldn't allow it. It may be that the water pump is KIA. If your engine temperature gauge runs "hot" (over 100C) and turning the heater and the fan on lowers the temperature and keeps it lower, then that would prove that the water pump is knackered. You'll need a new one (and also fit a new cam-belt.. job done for another few years).
If you have a duff water pump, you won't get gas in the rad' unless it boils again... so if you keep seeing gas, then it's HG. No gas.. but running hot (and the fan/heater helping to lower the temperature) is the water pump.
Finally while you're there, check the radiator itself. If you see any signs of coolant on the bottom rail, hoses or inside the plastic guard, you could have a leak which might cause any or all of the above symptoms.
Ralf S.