Best way to tackle this please, gentlemen?

[markDIY]

Im not a roofer, but do most other bit of building work competently myself.
This valley has been problematic for years I think. Some of the smaller tiles have slipped and cause obstructions with bits of moss and recently motar snots from 
next doors loft conversion. Im happy to scaffold it and get up there to do repairs and I suppose Im wondering what best to do with these tiles, the slipping ones and generally.
How can I stop them slipping and perhaps should I be trimming them back with a grinder in situ? Thanks for any advice.

[Redbeard]

Depends on how 'in situ' you mean. Cutting down onto the lead - no, unless you can slid something 'un-cuttable' underneath, between tile and lead. Dust extraction/suppression, perhaps easier on a bench on the ground, and no lead to cut/ruin. (I'm not a roofer either!).

[markDIY]

Cheers
Ive got no problem tucking something under the tile and cutting with small grinder and stick a mask on