This is not the only plant named Lanarth. There is also the very handsome Viburnum plicatum Lanarth, which has branches that rise in tiers like the arms of an overexcited cake stand, and a white lace cap Hydrangea macrophylla, called Lanarth White.
Lanarth is a village above the Camel estuary in Cornwall so I assume that both plants have a connection. Cornwall has some of the finest gardens in the country many of which house extraordinary magnolia collections. Because of the mild climate the magnolias are seldom hit by late frosts and are spectacular from late March.