Lawn help

Pinklepurr

Help! My grass looks terrible.  A couple of weeks ago I noticed whilst mowing the lawn that some odd irregular patches were forming and becoming increasingly widespread.  The grass blades were thicker and longer than some other areas.  In the last few days these patches have become dry, brown and matted looking and some of the leaves have red/purplish streaks on them. I’m at a loss as to what has happened.  Initially I thought it was just dry but the lawn survived the 2018 summer heatwave ok and didn’t appear anything like this.  We use Green Thumb and had the spring treatment done on 22 Feb (fertiliser and weed killer) and the lawn was aerated by them in Nov/Dec.  The garden is north facing on clay soil.  We don’t apply any other treatments to the lawn ourselves other than the occasional watering.  I used a spring tine rake to gently lift the matted areas to allow air to circulate but it almost seems as if the grass has died??  I’ve got green thumb coming on Friday to do their early summer treatment but ahead of that, I wondered if anyone had any bright ideas?  Pics attached.

Fairygirl

It'll just need reseeded in those areas, after scraping out the dead brown stuff.
North facing and on clay - always going to be harder for grass to do well without a bit of effort and maintenance.