Keef .. I impressed with the idea of rendering over corrugated iron. Who'd a thought it would stick. Perhaps that's why he used rusty.
I you have a search around the net, I'm sure you will find information on the mix you need - with or without the addition of cement. I think you will find that you will have to soak your lime, whether or not you need clay (and that needs to soak anyway) .. but use it straightaway once you have put the cement in.
It is my understanding that cement isn't needed anyway.
I used a mix of only hydrated lime and sand (soaked overnight) for interior patching on old (cementless) mortared bricks. Just out of curiosity, I used the leftover to patch an exterior wall where a pipe had been taken out, and added a couple of bricks to a garden wall. This was done about two years ago and the exterior mortar shows no sign of deteriorating.