Help with house

Sorry to waffle hope someone can shed some light!
I have been divorced over 3 years now and live in the FMH which is in joint names with my 2 children one is now 18 the other is younger, my partner (Hate the word boyfriend hehe im over 40) stays quite a lot more now in the FMH, but still rents his flat. He gets on well with my kids so things have been good. I knew when I got a great promotion at work a few weeks ago my ex would have one of his tantrums (He starts making waves whenever things start to go good for me even to the point of me getting a new car he made a fuss!)
He only pays towards the mortgage instead of child maintainance (which he has now stopped paying for the eldest and had also reduced payments for the youngest as he claims he has had to take a pay cut and he claims he has spoken to the csa but his lifestyle and the figures do not match when I asked him to confirm his figures had changed from a few weeks previous!I don't go to the CSA as there are too many horror stories and a bit is better than nothing!)It equates to approx 1/3 of monthly payments.
I had a knock at the door tonight and a man asked me to confirm my name and said he was from a solictiors and wanted to confirm I lived there? I dont owe anyone money etc and he said he didnt know why he had been sent. Further investigations on my part shows he the sols company are linked to my ex husbands so im certain its him who has sent them. I tried calling them, but they were closed im a bit puzzled as to what he could be up to next? Has anyone any ideas?

Did he tell you who he was and identify himself?

Presume you told him to go away and mind his own business?

Do you think it more likely to be someone sent by your ex to see if your partner answered the door?