It sounds like a postie is trying to be "helpful" for some reason. This would be more likely to happen after there has been a redirection in place and a few things come through after it has finished.
The only option is to complain. It may be worth just going into the Delivery Office and speaking to a manager there, alternatively ring up and get a complaint sent to the office.
If you haven't set up a official redirection you need to contact all those important places eg tax office, DVLA, banks etc and inform them of your new address. The alternative scenario of doing nothing about it leaves you potentially open to fraud. It does indeed sound like the regular postie knows where your in-laws live and is trying to be helpful. Your best course of action is to get your address details amended with the important places ASAP!! Failing that pay for an official redirection. Complaining to the RM that you have not paid for an official redirection will not solve the problem.
Page 1 of 1