There is no legal or BSI requirement to test the CO detectors with a test gas. A weekly check with the test button is fully sufficient if you choose a quality CO detector. The test gas is offered as a 'belts and braces' option.
(doc:536 V1.0). Our articles are reviewed regularly. However, any changes made to standards or legislation following the review date will not have been considered. Please note that we provide abridged, easy-to-understand guidance. To make detailed decisions about your fire safety provisions, you might require further advice or need to consult the full standards and legislation.