I've found that in some recent poor summers, especially those with a dull/wet June, it is normally the late July period that brings the sunny warm weather. Last year and 2008 spring to mind, although I would like it to continue in to August this time, unlike in those years.
Always nice to have any sunny spell running over the weekend for a true summer feel, out in the garden with friends and a few beers. Hopefully we get that because it will be autumn before we know it.