REDSAW,
I hope you are right but not right, I did think of saying what you just did originally, but thought
no plumber would fit the cistern so it was not sitting on the pan square front to back, it's the first thing I do, just let the cistern sit there before tighten the screws, go look down the back of the wall and then under the mating edges, when I can see it's square only then tighten the wing nuts, if he comes back and says your right, well Boyle s and Charles Law and all the psychometric charts need to be re-written or I'm not a plumber.....and the first law of physics and I forgot plumbing is not going to be declared a trade next week after all. Another way is to assemble the pan and cistern before you slide it all back into the pan connector making sure when the pan is in the socket the cistern just touches the wall, look in the video below. 2 mins in if you don't want to watch all the easy bits. This is all very well unless you have a very heavy pan and cistern, I fitted a Cooke & Lewis pan and cistern at home 4 weeks back and you couldn't do it this way, much too heavy and you need to lift the pan over onto the floor brackets.
Installing a close coupled flush valve \ push button cistern - YouTube