Although I still need to get to the bottom of the numerous records in the lock_db table, the probem of adhoc tasks not running has been solved.
I'll post the solution here in case anyone else finds themeselves here with similar problems.
There was a user data request (core privacy plugin) in the queue (task_adhoc table) which the adhoc task runner was getting stuck on. Running the adhoc_task.php script (/admin/cli/adhoc_tasks.php --execute), showed clearly where this was failing. Deleting the request directly from the task_adhoc table allowed the task runner to continue, and only took a couple of hours to process 13k outstanding tasks.
I'll post the solution here in case anyone else finds themeselves here with similar problems.
There was a user data request (core privacy plugin) in the queue (task_adhoc table) which the adhoc task runner was getting stuck on. Running the adhoc_task.php script (/admin/cli/adhoc_tasks.php --execute), showed clearly where this was failing. Deleting the request directly from the task_adhoc table allowed the task runner to continue, and only took a couple of hours to process 13k outstanding tasks.