I'd be surprised if they don't know that I'm married, considering I've mentioned that I'm married on every tax return for the last nine years, so they could send me a form with all the stuff that they do know about and ask if they need me to correct anything, or if I have anything else to declare. They could ask "Are you still married? Are you still married to the same person?" and update stuff.
It might be that if they sent you the "here's all we know, please add anything else" thing, people would not add things that costs them more tax, safe in the knowledge that they don't know about it.
