In message , Donwill
writes
It's understand it's a cross S. Retusa x S. Myrsinifolia and is
described as a small prostrate shrub, probably suitable for the rock
garden, relativly fast growing producing attractive fresh green foliage
and effective ground cover. (From Willows The genus Salix, by
Christopher Newsholme)

According to the web there's (at least) two plants going by the name S.
x cottetii; the prostrate shrub you mention, and a large shrub growing
to 8-10 feet. I'd believe that the name correctly refers to the former,
but when the OP describes the plant as a tree one wonders if that is
indeed what she has.
--
Stewart Robert Hinsley