What channel are you watching the previous seasons on? If it's anything other than The Roku Channel, you would need to ask whomever provides that channel. If it is TRC, be aware that when Roku licenses content for streaming, they might be restricted by the content owner as to the seasons they can stream, and sometimes even individual episodes.
Since you mentioned it's a British series, perhaps it's available on BritBox. I know my sister-in-law loves watching content on that channel, and they do seem to offer most of the available British programming. They offer a free 7 day trial, although I don't know if the trial restricts access to any content.
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Other than Season 1, many of the free channels/apps including the Roku Channel, Tubi, Plex, Filmrise, and Fawesome are showing seasons 2 through 8 of Heartbeat. Freevee is carrying seasons 2 through 18 for free.
Roku Community Streaming Expert Just another Roku user... I am not a Roku employee. Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102
I found an app called daily motion that appears to have it. The app is not on roku so you'll need to watch on your phone or PC. I just found it and haven't started watching it yet but it appears season 14 is all here and free! This is a great show! You would probably lime the Darrelles in corfu as well🙂