When you say "repaired ceiling" ,what do you mean? Is this a plasterboard patch that has been put in or what? It is a work of art trying to repair an artex ceiling and no matter how hard you try, it will be noticed for ever more. I just thought I would mention that. Also, the tub of stuff you bought from B&Q will be different obviously, from what was put on originally. It will probably be a thicker consistency. Whenever I repaired a ceiling in the past (stippled artex) and it was ready to be textured, I would mix up the artex (powder mix) and thin it down, and then using my stippler, test a bit on my plasterer's hawk, holding it upside down, to see if it gave me a similar texture to that on the ceiling. Sometimes it would be too thick, giving too peaky a finish, so I fiddled with it and thinned it down to get it similar to what was needed.The kind of finish you have looks to be on the watery side. Anyway, when it comes to putting it on, use your "texturing brush"(t/brush). When you have the mix to the right consistency, just "dip" the t/brush into the mixture, and stipple it on as you go, keep dipping/stippling it on that way over the whole patch, you can blend away the edges much easier into the original stipple. If you "brush" it onto a patch and overload it around the edges, it will make the patch look even more obvious. I'm not too sure about putting it on over 3 coats of vinyl silk, you will need to read the instructions. I suppose if it's just a light stipple, it should be ok.(anybody.... any other opinions). Don't forget as I said earlier, to stipple it on rather trying to brush it on, and test the texture on a piece of board or something to compare like for like before you do the job for real. You can see now why this type of decoration is no longer in fashion. It's a complete nightmare to sort out when it goes wrong.
Good luck.
Roughcaster.