Losing antifreeze from the overflow tank

I suspect you have a head gasket issue. You need to start with checking for a blown head gasket using the attached guide.

https://www.2carpros.com/articles/head-gasket-blown-test

Let us know what you find. There are a number of other causes but those are less likely and are normally accompanied by other issues. Is there any history to this? Did the engine overheat at any point?

No, our car has not been overheating.

Okay. That is good but doesn't rule out a head gasket. Let's run the test from the guide above and rule it out for sure and then we can go from there.

I'm not sure my husband can do all this as he's disabled.

Oh, okay. We will help as much as possible but unfortunately this is not the easiest work to do. If he is not comfortable doing it then it may be best to seek some assistance.