Posted: 11/08/2015 at 07:39
What sort of treatment was it and how was it applied? Was it exactly at the rate in the directions? I've a feeling the lawn may have been scorched. Was it applied in hot sunny weather - that can cause scorch too. Had the lawn been left to get long, then had it recently been cut short and then the treatment applied? That might be another possible cause of scorching.
I would give it a good scarifying with a lawn rake - this sort of thing is fine http://www.wickes.co.uk/Wickes-Lawn-Rake-1625mm-Carbon-Steel/p/500103 , wait until some rain is forecast in September and then scatter some grass seed all over the lawn -with the combination of warm soil and autumn rain and the grass seed will germinate quickly - don't mow it until its 3" tall.
Applying the lawn seed all over rather than just the bare patches means that even if the grass is a slightly different colour to the existing, it will all blend in.
Never cut the grass short - it just causes the grass to dry out and then you get bare patches where weed seeds can take hold. Far better to cut it medium but slightly more often - that way it'll always look healthy.