Insurance? That nice old guy can easily turn into that old numpty. I wouldn't pressure wash slate or rosemary tiles, you'll only cause yourself problems.
What are rosemary tiles?
Clay, slippy as poope to walk on
I would have thought you'd want a crawler ladders, soft brush to get the general crap off, and then spray the algo clear and let it do its thing (from what i gather, you wont get instant results using algoclear this way but it will come good in a few weeks when it takes effect)
Personally mate, i know you trying to do a good deed but slate tiles are an arse, slippy and fragile. Chances are you'll break one if not careful, and if your really not careful you'll break yourself
Mates away this weekend, i'll ask him when he's back if you haven't already sorted something