I have a block retaining wall approx 1.5 metres in height and 12 metres in length. It was rendered and then painted. The paint and even some of the render is now flaking off (in some areas through to the scratchcoat) I suspect the blocks have no membrane down the back of them or a suitable pebble/stone filler before the soil. Can I spray the wall with a suitable material that saves me having to paint it again (which would only come off again with the moisture) or will I have to dig out the soil from the back of the wall down to the foundations (horrible job!) and put in the correct membrane and stone drainage system.

I really don't know of any product that keeps moisture from seeping through a retaining wall if the wall isn't very well backfilled with drainage stone. So until you know what's behind the wall, I would just keep wire brushing off the peeling paint and render.
I bought some very expensive foundation sealer (clear) for inside and outside walls, as well as slab floors. Such sealers can be bought tinted. But they don't really hide the concrete and aren't meant to, but at least they can't peel off.

You could contact rockcote.com. They will answer questions. I just found them on a search and have never bought their product.