Originally Posted by Helen R

It sounds like it might be Fusarium Patch Disease which is quite common in the UK at this time of year. It occurs when the weather is relatively mild and humid and will cause yellow-brown patches to appear. However the mycelium are more like cotton wool or cobwebs than mildew. As far as I'm aware fungicides aren't available for use on domestic lawns. The effects can often be short lived and the disease may return again as soon as conditions are suitable. A sharp frost will check the disease naturally without resorting to using chemicals.

Applying nitrogen will exacerbate the problem, so if you do need to feed then make sure it's an autumn/winter food which is low in nitrogen.

HTH