Halifax sending letters/statements with incomplete address details

Have 2 accounts with Halifax. They have one line of my address wrong. Thought I was still getting the post, but just seems to take a bit longer. Have complained since Feb. - Got a letter in March saying it was fixed, still getting letters wrong in May. So obviously it is not.

Found out yesterday that the fixed term on one of my accounts expired in Feb and they send you a letter before putting you down onto the lowest interest rate. I never got that letter. Because clearly it has gone around the houses somewhere.

Am I entitled to ask them to credit me with the interest I should have got?

Tell them that you are going to complain to the Data Protection Commissioner if they do not sort this out within 2 weeks and demand a letter of apology from the Halifax, and an ex-gratia payment for all the hassle they have caused. The last thing you want is your details going to someone else!!!!