He got four on his leg behind the knee. They've gone hard and dark purple like really nasty bruising and he's having trouble walking. Some have burst and are weeping. I don't think he's been scratching them too much, but maybe in his sleep. He does often get a bit of swelling from bite, but not like this. He asked me (in tears) for a note for PE today. It's the first time he's ever asked in 8 years at school. 
I spoke to NHS direct and was "helped" by a man with heavily accented English which I struggled to understand and who had never heard of gnats 
He said I should take him to the nearest minor injuries clinic which (he told me) is 3 towns and 10 miles away. There are numerous GP surgeries and clinics much nearer, including our own.
He's well in himself and doesn't have a temperature, although the bites themselves are very hot to the touch. WWYD? He's away on his Yr6 residential at the weekend, which whilst not relevant to any treatment he may or may not need now, is playing on my mind.