KBellamy wrote:
I moved into a new house last year and had my garden landscaped and
fully planted,

(snip)

Get back in touch with the Landscapers who have planted your garden.
You ought to have had, as part of your contract, a snagging timescale,
be it 3 months or 6 months review. As planners/architects we usually
give 6 months. It sounds to me that you have some serious drainage
problems but perhaps that some of your planting has had a shock and
therefore they need to be replaced. I am very sorry for you to have to
witness all this, which certainly could have been avoided. Also it
takes years and years to create a garden )