Brief history: DH was going to take my surname when we married. ILs objected so we both had sep names until DD born in 1986 when DH changed his name by deed poll to Myname-Hisname.
All our legal stuff is under Myname-Hisname for all 6 of us, but for day to day life we all tend to use Myname. I was quite shocked the first time I overheard DH on the phone saying "this is Mr Myname" but for the past 17 years at work DH is known as John Myname, just as I am Anne Myname. DD hates her father's surname with a passion and when she registered for 6th form did so as Sally Myname.
(Sorry, not so brief- are you still with me?)
Most of the family on both sides have managed to cope with this and send things to us as Mr & Mrs Myname-Hisname, even the ILs. MILs sister, however, has continued to use just Hisname. TBH I'm not bothered when she sends stuff to DH under just Hisname. It's when I get things from her addressed to "Mrs Hisname" that I get wound up, and just lately there has been a lot of it. She also addresses DD as "Miss Hisname" and she is getting really annoyed about it too- as DD says, this person doesn't exist!!
I would also mention that it has taken her until this year to spell my first name right. She usually writes Ann.
Leaving aside the ins and outs of why it shouldn't annoy me, I've got to write to her for something else and I want to take the opportunity to mention the fact that I would prefer her to use our proper names. But I don't want to upset her, and the last person I wrote to took offence . Can anyone suggest some tactful wording that I could use that she can't possibly misinterpret, that says clearly that I want her to use the right names but without being rude?