Data Protection

My ex husband of 20 years recently received letters to his address ( he has only lived there for 4 months) & I have seen him once in 20 yrs he has remarried as I had too. I didn’t know his address. He returned them to sender unopened.
By pure chance my 29yr old visited him & he gave her a further letter addressed to me unopened. When I read it dated 28th May 2019it was a reminder letter about a questionnaire they had recently sent on the 9th May.
I’m extremely alarmed as I was a victim of DV & find this very distressing.
I have phoned the contact number on the letter which is for an Newcastle department who cannot give me an explanation as to why this has occurred they failed to recontact me as promised within 3 days & I had to contact them again today & still they had my address as my ex husbands. My job centre is in Carlisle & I have lived at my Carlisle address for the past 13 yrs .
It was suggested that the child support agency had updated the new address but my youngest child is 29 & in any case he never gave me any financial support for her whatsoever.
I have no idea where to go From here but I feel I should have a explanation.