The environment Agency advises septic tanks should be emptied annually to ensure that your system is running efficiently and is not polluting the environment. Over time, waste builds up, which can lead to backups, unpleasant odours, and even system failure. Regular emptying also protects the drainage field from clogging, avoiding costly repairs and environmental harm.
Please see the below links to find out your obligations https://www.gov.uk/guidance/general-binding-rules-small-sewage-discharge-to-the-ground#overview& https://www.gov.uk/guidance/general-binding-rules-small-sewage-discharge-to-a-surface-water