They are not using Morte De'Arthur as a basis.
In the tv programme, Merlin is only a young man, and I'm assuming, younger than Arthur.
Uther is still alive at the moment, and Arthur is trying to prove his worth as the future King, along with Merlins assistance, but as of yet only one other knows of Merlins powers, the court physician, played by Richard Wilson.
It's quite good as so far as early Saturday evening TV goes, with a different story every week. I enjoy it anyway.
:-)