Roofing hip dormer

[Dee]

Hope to be tiling the roof next weekend but a few questions first.... 

I'll be Reusing the concrete plain tiles with tiled valleys and bonnet hips (when I can find them). Both roofs are 45 degrees. 

Do I put anything where the cheeks meet the original roof? 

What sort of valley tile do I buy (Angle)? 

I'll be cladding in hardi linea,  so again what happens at the cheeks /old roof junction.... Does the cladding just sit on top of the tiles? Do I leave a ventilation gap.... how much? 

Apart from the roofing membrane does anything else sit on the valley boards?