The message
from "Brian Watson" contains these words:

Alternatively, take the advice to build up some parts and lower others and
then follow my previous advice re draining it from the lowest point.

Personally I am wary of this "you may have a case..." thing.

Litigation is a pain in the wallet and you can easily turn this situation to
your gardening advantage.

Litigation isn't necessary; there are trade and planning standards
which housebuilders are meant to conform to, before the building's
completion certificate and construction guarantee can be awarded. The
building must pass planning and building control standards to satisfy
building insurance companies and mortgage lenders. Regardless of whether
the builder is responsible for landscaping the back garden, he's
responsible for surface water drainage surrounding the house. Three
inches of standing water throughout winter on a flat site might breach a
dampcourse or rise up under floors. The OP can take it up with the
builder (in writing), and then the planning dept, local building control
etc. Ultimately, you with-hold payment until the building meets the
legally required standard.

Incidentally, you can only drain a garden from its lowest point if
there's somewhere lower, and legal, for the water to go. Getting that
wrong (piping it into next door's garden for example) is far more likely
to involve the poster in litigation, than making sure the builder meets
normal contract standards.

Janet