Zarch said:

As you can see from the pictures that the problem is mainly in the
front left portion of the garden.

This has part of garden has plenty things NOT going for it.

1) this is where i walk to get to the shed and to water the border
every other day
2) this part of the garden in virtual full sun throughout the day
3) I believe the roots for the large Silver Birch tree are right under
there
4) i've only created the border this spring and so this part of the
garden has really took some hammer under foot.

I think between #1 and #4 (with a bit of #2 thrown in) you have an
explanation. The soil is compacted and the lawn stresses out in the sun.
(#3 isn't helping the lawn at all, either.)

The solution might be to core aerate the trampled parts and then spread
a top dressing of compost over it. Watch for water stress and maybe
give it some supplemental watering.

--
Pat K. ('someplace.net' is comcast)

Any technology distinguishable from magic is insufficiently advanced.
(attributed to Don Marti)