TL;DR: What non-fall-arrest-requring ways would you suggest cleaning the exterior of the fixed half of a sash window?

Hi folks,

I've finally outgrown the flat opp. Summerhall, so I'm moving! This means I need need to tackle the elephant that is clean my windows. 

In normal non-gestures at the whole situation times, I'd probably just harness up and borrow the appropriate temporary fall arrest gear from work - perhaps buy my upstairs neighbors a case of beer in return for letting me put a pair of parapet clamp anchors on their window sill, or a doorjamb anchor, or something similar.

However - there is a pandemic on and it's been a year since I've done *any* work at height, let alone actually doing any complex ropework, so I turn to the ehlab-and-adjacent hivemind - how would you attack this problem?

Cheers,
A