Well it said on the email it was the last night to enter.  But on the entry page it says you have until August 6?   So check with Sky TV.  But if you want a trip to Croatia then Sky TV are giving away a trip to Croatia for a Game of Thrones tour.   Here

From Sky TV:

Enter into the world of the Seven Kingdoms on a 7 day Game of Thrones inspired tour in Croatia. You’ll visit the Kings Landing palace, sail to the House of the Undying in Quarth and many other majestic scenes filmed in Dubrovnik, Lokrum and Trsteno.