Hi All Can anyone offer some insight on how this task is routinely done on sloped roofs- cleaning the outside of the stack - apart from using scaffold or MEWP. Any thoughts appreciated Thanks

In the dodgy world of assumption, i reckon you mean domestic chimneys. You can have cantilevered scaffolds giving direct access to said chimney to prevent a fall or having a tied scaffold at the eaves/gutter level to protect the people. MEWP'S also a safe way of access. Other choices? Ladder then Cat Ladder but that means carrying a bucket up both ladders. Harness? What could you attach it to? The chimney? Might not be strong enough. Stick with Plan 'A' imo.