Once I have rolled the wall, once it has dried, I can see the roller lines. This is a hot bedroom in a ground floor flat. Any ideas?
I suggest two reasons. One caused by the other. You mention the room is hot. This leads me to believe the paint is dying quickly and helping you leave the roller lines. Ultimately, what is the underlying cause is you are not rolling with enough paint on you roller. In professional terms, "dry-rolling."
What we mean by this is you are either not putting enough paint on you roller to go to the wall, or you are trying to make one roller full of paint go too far. Try the link on Karl's website "Interior Painting" for some suggestions to remedy this situation.