Yellow leaves falling off on cherry laurels

I purchased 6 x 80L potted cherry laurels (2M tall) which were planted at the end of April.

All has been fine but suddenly over the last week and a half I have witnessed noticeably more leaves turning yellow and falling off in greater quantities.  I suspect this is due to the recent arrival of hotter weather and thus is under watering (albeit I have been watering twice per week and am aware if the need to water often). 

I have attached some pictures as I need some help to ensure I have correctly identified the issue and ensure that watering more often than I am currently doing is the solution.  For context the plants are in a south facing garden so get lot of direct sunlight.

I noticed that the area around the stem from the original potted material is quite heavily matted with smaller (feeder?) roots and has less in the way of soil content. I am assuming as a result this area is less effective at retaining water and may need more watering (every other day?) or even additional soil/mulch to stop the roots drying out so easily?

I also want to eliminate it being something else like iron deficiency and check that the plants have been planted to the correct level.  It looks like most are flush with just two of them standing proud of the backfill by maybe an inch or two (not sure this makes any difference as they are all experiencing the yellowing).

Any advice would be much appreciated.