they do get cracked heads, i have personal experience of that
Im still having overheating problems with mine when its sitting at idle for any length of time.
If im moving its fine unless I come to a long hill and then its overheats, well it starts to, temp gauge goes to 3/4 and then comes down when Im on a level.
Im waiting on a new rad from LR at the moment as I think the aftermarket one that was put in was damaged when the head went. I came to this conclusion because if I checked the hoses going to and from the rad with an infrared thermometer they were reading almost exactly the same temps which cant possibly be right in my opinion.
On researching how the radiators work there is supposed to be a plastic baffle in them so the coolant cant just flow along the top of the radiator and has to travel through the radiator before leaving. I think this baffle got damaged/destroyed when my head went.
So thats what Im waiting for at the moment, a new rad from LR, I just hope it works as im out of ideas otherwise.
As for your problem, are you SURE its overheating?? What are the temps of the hoses to and from the radiator?? I know mine is starting to overheat when the infrared thermometer read anything above 86/87*Cish.
Are the hoses hard after you have been driving?? If so then you may have a hairline crack in the head like I did that would only cause the engine to overheat when it got really warm. In my case after around 15/16 miles @ 80MPH on the M40. I came off and started along a dual carriageway and thats when I started overheating. I stopped almost immediately but upon checking the rad hoses they were rock solid and coolant had been lost from the overflow pipe on the header tank.
If the hoses arent rock solid and you can squeeze them without too much resistance I would check the temps of the hoses. The dash temp gauge could be lying to you and all you need is a new coolant temp sensor. Although if you say theres air in the system then I would do as others have said and get a sniff test.
Sorry if this is bad news, I know how I felt when I found out I had an expensive problem
-Wills