I'm not sure on the layout of a Freelander engine bay but usually the header tank is the highest point in the water system, so if the header is empty but the pipe just underneath has water in it and the water system is still full then it must just be the header tank. Replace it and see if that cures it. And don't use radweld. That's for people who really can't be arsed to do a proper job and find the leak. However, if it's not the header tank that's leaking the leak could be from anywhere, but in a positive way you see a puddle on the floor so you know roughly where the leak is. You can get some additive to go in the water system that allows you to see leaks under a UV light. A little expensive but can save you a lot of time. Failing that cover the area above the puddle with talcum powder as this will show the track the water makes. I did that on my 110 and I found the culprit pretty quickly.
I am quite sceptical when people say 'I've just had XXX fitted so it's not that'. You should see the amount of threads created when people say that they've just had XXX replaced and they're still having issues. Faulty parts, parts fitted incorrectly, people forgetting to tighten stuff up etc are also things to take into consideration.