The information about this is in the back of the old style resident permit but I believe that the time scale for doing so has been reduced. Delaying informing the authorities incurs a fine. Not sure what the costs are for the old permit.
New resident permit holders will be please to know that for them the information is just taken (at the moment) by the Passport Police and I believe it's entered into the computer and it's done there and then, no need to send off the permit any more.