Posted: 08/07/2017 at 17:56
I would be patient ... the grass seed may not take because of the presence of weedkiller - some lawn treatments contain pre-emergents to prevent weed seeds germinating ... afraid this affects grass seed too. However, it also contains fertiliser, so that will encourage what grass you have left to thicken up and spread.
I would then leave it a few weeks for the effectiveness of the weedkiller to wane and to get past the hottest months, then in September I would give the lawn a thorough scarifying, then a good watering and sprinkle some grass seed all over. Try to time it when the weather forecast is for a couple of days rain. Grass seed germinates really well in September, it's the ideal time.