You have the right to complain via letter, but they have the right to attach your wages if you don't pay it. Welcome to IRAS. They, like the IRS in the US, are the strongest single body of the Government. They are also the most efficient bill collectors out there as they can and will garnish your wages if you do not pay.
It sounds like your landlord is retired and therefore all his income is via investments. As you are furnishing him with income, they have legally made you their agent. If you look at the letter, it may well say from the Comptroller of property
Tax instead of Comptroller of Income Tax. They also collect the property taxes and the TV license fee. It is entirely within their power to make you an agent of collection.
I received them all the time as most of the locals we employ are low-wage deadbeat types who don't bother to pay their property taxes and TV licenses (both come on the same bill in January). Usually, I have to write a letter of objection back as the individual names has left our employ some years earlier.
It's official and it's wise to pay them as requested. You don't want to run foul of IRAS.