Janet, I think you are probably referring to the hybrid Halimium x
pauanum. True Cistus are either white or pink flowered, which is
possibly why you you couldn't find any reference to it under that name.
Anyway, if you've got the right plant, its a superb thing massed with
flower, growing to around shoulder height and possibly a bit more
across. The flowers are always a bright clear yellow with no markings.
It is as hardy as you could wish for and rarely takes a 'knock' in the
worst of winters. I had it a good while ago and almost needed
sunglasses to look at it! Unfortunately, there are plants marked as
this that are pure Halimum halimifolium, which is highly variable, but
almost always an upright grower, higher than it is wide. This is not
to detract from the species of course, merely that the hybrid makes a
more pleasing rounded hummock.