What is the best route to take when mapping an entire organisations processes. Should this be done by department/business unit or should this follow a service based structure and create end-to-end processes based on services?
Grouping by customer interactions / end to end process is a great idea as it really aligns teams around the customer rather than 'my department'. It's actually easier to do this than you'd think, and doesn't require any restructure. You can start by creating your desired groups, and then simply shortcut processes in from anywhere in your site. That way, you retain both navigation options (departmental + end-to-end) so teams can navigate via their preference. You can even create end-to-end overview flows (at the group level) without moving processes out of legacy groups.