Ex keeps on using my home address

My ex moved out of the family home April 2018 to move in with his girlfriend pretty much soon after. He is still using my home address as his - even for his new job that he started in May this year. It has come to light when I opened mail and realised it wasn't mine, but his. His credit card statements still come to my address and more recently a credit agreement for a new tablet also came through the door.

He is in a bit of financial trouble, and currently we are only linked on the mortgage - which I'm in the process of moving to my name (I can afford to take the mortgage over and essentially buy him out). I am concerned as to why he keeps on using my address......his girlfriend lost her benefits when he moved in (or so he has told me!) So I'm now thinking that she has been claiming benefits all along - my children go to their house every second wekend and for school holidays, and now I'm worried that her lease hasn't been updated either - she is currently living in a council house. I also claim the 25% discount for council tax for single adult and tax credits to help with the childcare for my kids. With his mail coming to my address I could be investigated for fraud.

I have spoken to him numerous times around using my address - I'm going to pick it up with him again, but am thinking that my best bet might be to go through a more formal route? He seems to be quite willing for me to be investigated for fraud, which will affect his kids ultimately as I am their main provider or have debt collectors come knocking on my door as he has been missing minimum payments and defaulting on his loans.

Any guidance would be greatly appreciated. We are in the process of getting Decree Nici sorted - with the financial settlement pending as well - on the outcome of the mortgage agreements etc.

Simplest solution would be to mark everything that comes "NO LONGER AT THIS ADDRESS, RETURN TO SENDER" and pop it n the post.
Don't even bother to mention it to him he clearly isn't going to stop if you have already asked several times.

If any debt collectors etc call just tell them his new address and that he moved over a year ago.