Thousands of people flocked to Stonehenge to mark the longest day of the year - known as the summer solstice (pictured left). The event opens the way for another heatwave to reach UK shores in the next two weeks. Every year the neolithic Wiltshire monument becomes the centre of day-long celebrations to mark the official start of summer and up to 25,000 people have attended the site so far today, as the sun rose at 4.52am this morning (pictured top and bottom right). And now with summer well and truly upon us, the Met Office is expecting a steady increase in temperatures over the coming days - with up to 81F alone expected this Sunday. Experts expect it will be even hotter into next week.