it is possible you have a fungal disease causing the issue. These are more prone in warm wet weather such as we have been having this summer.
Red Thread is a common one found which can be treated with fungicide and also giving the lawn a good feed and adequate drainage will help.
Another possibility is a lawn pest such as leather jackets or chafer grubs which eat the roots of the lawn and kill it. These can be treated with an insecticide.
Thanks hawkwalk for that information, could well be an insect as the birds spend quite a lot of time pecking on the lawn,will have to get some insecticide.
Thanks kenrick yes we certainly have cats entering the property as is proven by the heaps left on the garden,despite having a cat scarer & curry plants which are supposed to deter them. Have tried the insecticide idea but its early days yet.