I shall be re-rendering a stone wall that has been previosly cement rendered and is showing many hairline cracks as well as an area which has completely failed and the render has blown and bulging from the wall.
I have read from previous posts that lime to be added to the cement render top coat to aid flexibility and reduce the chances of cracking.
should lime not be used in the in the scratch coat too and should you still use plasticiser and waterproof additives to the mix?
what mix ratios for scratch and top coat are required?

I will move this to plastering forum, you may get more response there.

Sounds like the render didnt bond to the wall, hack it all of and give the wall a SBR slurry, paint it on the wall and apply your scratch while the slurry is still tacky, with water proofer and febmix, at a ratio of 4 sand to 1 cement.
Next day float coat at 5 sand to 1 cement, add febmix, no waterproofer.