waterproof render?

Hi, after moving into our new house I thought I had inherited quite a nice workshed. Breeze block built, felt roofed, rendered and external paint. I've been doing my woodworking projects in there.
The trouble is, this weeks horrendous weather, rain rain and more rain and driving gales have been forcing rain through the walls! The paint has cracks in it and rain has been driving into these cracks and coming through the render and breeze blocks where they are mortared together. There is a right big pool in the middle of the floor.
I'm guessing this isn't just a case of repainting, but a case for re-rendering? Just what materials and ratios should I use to make sure my job is waterproof? Should i have 1 or 2 coats?
Any help gratefully received.

Is the outside painted and accessible ?

Hi, yes all of the building is accessible. The outside has been painted in a coat of textured external paint. This has bubbled and cracked and that's how water finds it's way onto the thin render and through the wall.