another disco II overheating

I have a 2001 discovery II that started overheating yesterday. It appears that the water is pushing back into the bottle and flowing out the over flow hose. I put about a gallon and half of water back in and started it up and once it got warm the water started boiling back in the bottle and I immediately cut the engine off and let it cool down. I noticed that with the heater on hi that the air was cool.
 
I have completed the first step and that is not to run or drive it until I figure out the problem. I plan to have it towed to my mechanic but I was wondering if anyone thinks the thermostat may be stuck? I guess it could be several things but I am trying to be optinistic. Also, is the thermostat behind the alternator and air compresser (the long metal pipe)?
 
 
Thanks,
 
Randy 

Since you have a cold heater you have a lot of air trapped in the engine''s coolant system. What led up to this, have you been having to add coolant?

Yes Mike, I have had to add some coolant from time to time. There have been a few times that I have noticed a puddle on the ground under the passenger side of the vehicle and it was a big puddle. Is there a way to id if a head gasket has gone bad, visibly or do you have to do a leak test? How do you remove the air from the system?

Go have a shop do a coolant pressure test to find the leak.
PM me your tele number and a good time for me to call you.
I wouldn''t drive it till we talk or you get it fixed.

Sounds like you need an engine