Why has my new lawn got brown patches

could just need extra water because of the heat,you could also give it a lawn feed which might help situation, i was advised the same and it seemed to work,hope this helps and welcome to goy,though one of the experts might comment shortly

Is the lawn completely flat, or have you got low spots and high spots? If you have these, when you cut, you shave the top off the high spots, scalping the turf and creating brown patches.