Did you scratchcoat the trial area first? Normally with cement render, you would scratchcoat the area first, then leave it for a couple of days or so. Your mix would be 4 to 1 plastering sand and and cement, with an added plasticiser/waterproofer. This should be added to the mix in a "measured quantity", as specified on the container. Once the scratchcoat has set and cured, (1 or 2 days), you are ready to put on the top coat of render.
Once again, use plastering sand, and this time, the mix would be 5 of sand to 1 of cement. Again, use a measured amount of plasticiser/waterproofer in the mix. Choose a nice dry day, not too sunny, and about half an hour or so before you start, give the scrathcoated wall a slightly wet/dampen down, just enough to take away any suction in the scratchcoat. Once you're ready, apply the top coat of render, keeping it as neat and flat as you can. Because it's just a small wall, you wont need to wory about using a straightedge, although a darby would help. Once it's all on, leave it to tighten up. After half an hour or so, touch the render with your fingers, you'll feel it getting firmer, you'll know the difference. Don't fiddle with it too early though, otherwise it will "slump" or move about as you said. With experience, you'll know when it's ready to float up, by the "feel" of the render. Go over the render once with the float, then leave it a while. Let it go off a bit more, then go over it again with the float. Keep going over it with the wooden/plastic float until you're happy with the finish, the look of the surface, and the eveness of the render. Once the render has been completely floated up, then use a "clean" sponge", and "gently" rub the render in a circular motion, working across the wall starting from the top. The sponge gives the render a nice, even looking finish. Always remember too, that the render at the top of the wall will tighten up/dry out, a little quicker than the bottom, so always dponge the render carefully. Another very important thing as well, is to keep your sponge CLEAN. Give it a good wash out every couple of minutes in clean water, and wring the sponge out well each time.
Give it a go Adrian and see how you get on. It's hard to explain in words, (easy to demonstrate), but with a little time and experience, plus learning from your mistakes, you'll pick it up.
