First off, thank you very much for the helpful replies! I didn't think I could actually file for this but am happy to give it a go, especially as the IRS sent me a letter and say they will let me know why I don't qualify for it!
It is my understanding (and my understanding of US tax code/law is quite limited, just asked a few questions on here!!) is that I made a mistake with regarding how I filed when I first started filing as a married person over here 10 years ago. I didn't realise that I could file married separately, so I got him an ITIN, filed jointly and then did this. I understood that once I've made that choice, I have to do this or go through a somewhat difficult procedure to stop. From what I read recently, as I never formally agreed to do this (send a letter to the IRS saying why I was doing this?) there was some legal case where I supposedly could change back to filing separately, but I was worried that this might flag it up and I didn't want to get audited or something. The more I type, the more I think I probably should have spoken to someone.....arghhhh......
Our income will never be over the threshold unless we win the lottery or completely change our chosen professions, so hopefully I'm doing this okay!!