The registered office of a limited company doesn't have to be where the business operates from. Many major international companies have their registered offices with, for example, small solicitors' firms hundreds of miles from where the company's offices are.
However it IS a legal requirement that the registered address of a limited company must be a place where legal documents can be served upon the Company Secretary, with the certainty that he'll receive them. (Since you've not entered into any agreement with your landlord, or his mother, to pass on mail that clearly isn't guaranteed if mail is delivered to your home).
It might just be an oversight. The business might have been registered to your address when the landlord and/or his mother (or another family member) lived there. If routine mail was actually going to another address it's possible that they completely forgot that the registered address of the company was where you now live. (I've done that myself. I ran a limited company with my father's home as the registered address but all mail was normally sent, via a PO Box, to my home. When my father died, and I sold his house, I completely forgot to change the registered address of the company).
Speak to your landlord (and/or his mother) first. If you can't get things changed, contact Companies House:
http://www.companiesh...ntact/contactUs.shtml
Chris