I also received the same letter from them this morning.
My UB mortgage was my primary residence years ago. At the time it ceased to be my PPR, I informed them that it was changing to a rental property and they converted the mortgage to an investment tracker mortgage rate. So, all totally legit from my end.
It beggars belief that they now need to contact me to confirm this is the case. Have they no records??
I won't be replying to them.
I would like to know the reason for this "out of the blue" communication though.