It sounds like you're refinishing what we call here in the U.S., drywall. It's a gypsum material between two sheets of construction paper. Very durable and easily repaired, just as you are doing. I'd suggest, however, that you look for a spackling repair material that has fibers incorporated. This prevents much of the shrinkage. My favorite here is Red Devil but, of course, I've no way of knowing if it's available in the U.K.
More importantly, if your wall(s) is this kind of construction (I assume some similarities with the U.S.) then the "patch" needs to be finished once it's dried to match the wall. You should notice some texture on your wall. Many small lumps and bumps known here as "Orange Peel" or if they're larger sometmes called "Knock Down". This is the finish the installers put on the wall to cover up small imperfections. Again, here you can buy a small pressurized can of the material at DIY stores. It should come with three plastic "straws" attached to the can. They'll be color-coded for size of the lumps and bumps. Usually, use the middle size. Simply make sure the area is clean and dry and follow instructions. Don't hold the can to close to the walls and don't apply too much. Let dry and then re-paint. Nice thing is, if you don't like the first attempt, have a pail of warm water and rags nearby and just wipe it off. Don't sand it after it dries...
One other hint if you decide to go this route... don't try to confine the spray only to the area repaired... feather it into the surrounding wall...
Best of luck!