Must I reply to my lender?

I own two blocks of flats, each with different lenders with BTL mortgages on each block at rates around 1% over base and interest only.
A few days ago I received a letter from one of the lenders asking what repayment vehicle I have in place to pay off the loan, albeit there is over 5 years to run before the end of the term.
My monthly payments have always been met on time and I have an excellent credit rating, so I'm wondering why after many years as their client they should ask this question now.
Depending on circumstances/options I have at the time will influence the route I'll take but am I obliged to reply?

Thanks