The movie clearly states that it is the middle of the summer and supported by the fact that schools are still in attendance and the quote "It's the middle of the summer, its a heat-wave." However during the party scene, it is clearly dark outside but the clock on the wall as Jamie enters the house says 9:00. During British Summertime it would still very much be light at this time.
At the party, Leah is holding an object in her hand. If you look closely, you notice that the larger white end is in her hand with the gray thin end on top. When the shot is zoomed out, you can tell it's now upside down with Leah holding the gray thin end in her hand.
The rainbow early in the movie is inaccurately depicted: the red band should be outside the curve, and the blue/violet band on the inside. A rainbow is composed of the entire spectrum of colors of visible light, from the longest wavelength, red, to the shortest wavelength, violet. The order of colors in a rainbow is easiest to remember by the following mnemonic (a formula that helps one remember something): Richard Of York Gave Battle In Vain. R=red, O=orange, Y=yellow, G=green, B=blue, I=indigo, and V=violet. Red is naturally at the top edge (longest, or hottest in the spectrum) of the rainbow and violet is at the bottom edge (shortest, or coolest in the spectrum), with the other colors, in order, in between.
When Ste and Jamie are talking outside at the party, it's cold enough that you can see thin fog from their breath, even though it's supposed to be set during a heatwave. Occasionally, other almost-invisible puffs of fog appear behind Ste (probably from crew members), even though they're supposed to be alone.