How to fix this.

[cdbe]

As I'm on my scaffolding I noticed this cracked tile on my neighbours roof, which was done about 18 months ago. I think it was cracked when they did it (there are a few other shoddy areas for a pro job) but has taken a while to start moving down.

He's probably gone onto "My Roofer.com" so God knows what he'll get. I was tempted to help him out but I can't figure out how you hang a cat ladder over a hip - do you fix one hook on a lower rung?
And should the two kickers above be looked at?

 

[datarebal]

Ask the original guys to replace it

 

[cdbe]

Ask the original guys to replace it

It was done by previous owners just before they sold it, unfortunately I can't remember the name of the firm they used, otherwise as you say I would have thought reasonable to get them back.

 

[noseall]

Always an awkward one i.e. tile hip cuts on dry hip not sitting properly in the absence of benefitting from a mortar bed. We would always mix up a load of muck and bed the hip cuts as well as (try) and fix them to the batten. Couple that with the fact that there will also be a run of 50mm timber down the hip (to secure the dry ridge) and the fact that the tile will be cut to a point...
What you end up with is an unstable sitting tile that tapers to a weak point. The fitters should either mortar or foam these so that when pressure is imparted from the ridge, they don't unseat, just like the others are doing. I think this is one instance where foam or a bulky polymer adhesive would be suitable. We have been polymer adhesive gluing our Velux/Keylite slither tile cuts for yonks now.