So, having read more about the glue theory, and someone suggesting UV would help, I thought I'd try an ordinary lamp as I've not got access to a UV one.
I left my iPhone SE under a small bright light, and I think I must be going crazy, because it has somewhat dampened the yellowness! It's still there slightly, but it's no where near as bad as it was. I hope my eyes aren't deceiving me because I've been temporarily looking at the bright light and it really has made a difference, but I don't have high hopes...
If someone else could try the same, and report back.