Had our 15 degree fibre cement slate roof replaced two years ago on modernist 60's semi. Roofer since gone into liquidation (I know, plus he didn't complete the building control certificate). Straight replacement with new vapour membrane. Original sarking boards underneath. 9m rafter length.

Wind driven rain is causing leaks and I'm looking at options to get it fixed which I know is going to need stripping the whole lot.

Question is whether I go for a fix using the existing materials with a product like kwikslate or permavent easy slate, use 600x600 slates over all of it or just have it done in zinc?