In order to comply with Racing Rules of Sailing (RRS) rule 78.1, every boat's owner or other person in change shall ensure "... that the boat is maintained to comply with her class rules and that her measurement certificate,.....,remains valid."
For Mirror dinghies this means you have to have a measurement certificate for the boat with the correct owner's details on it. You also need to be a member of the UK Class Association, have sails that carry a certification mark and the boat's buoyancy needs to have been checked within the last 12 months.
The annual buoyancy test or inspection can be carried out by a Mirror Class Measurer , or a properly appointed Club Official. To do the test you will need a buoyancy tank pressure guage. If your boat fails the test, see this article on checking and fixing leaks in the bouyancy tanks.