I have a lot of brown patches on my lawn - the grass looks dead.

Can you go out and examine the patches closely - you're looking to see if there are any red threads, or small red needle like growths standing up from the leaves in the "dead" areas. More obvious when weather's damp, so now would probably be a good time!! This looks a bit like red thread disease (corticum disease); is your soil sandy? did you buy luxury turf?