Mortgage arrears charges reimbursement

I am not sure where to put this....10 years ago I owned 2 properties joint,y with my now ex husband. Both went into arrears during the divorce. Both sold and all monies owed paid off and I bought a new property and here we are.

Several months ago, out of the blue, I received a letter from said mortgage company saying charges were wrongly calculated and I would receive a cheque for the first property. Several weeks later, received same letter for the second property and a similar sum. Cheques arrived and banked.

Last month I received two ‘we overcharged you, you may be entitled to compensation’ letters which I binned. I don’t remember much as I experienced several traumas in a short space of time and frankly, the mortgage was the least of my worries. Binned letters.

Today I get home to another letter for property one, worded exactly the same as the first letter, save they are now sending me even more money!

Has anyone any experience of this? It is totally unsolicited on my part (although I did a PPI claim in January but not for this company as was no PPI on the account). Why might the money come in two parts? I will obviously call them to clarify but just wanted to know if anyone had similar experiences. It is a considerable and very much welcomed sum of money but I feel I need to pinch myself. How/what/why/when?!