John, Clematis 'Sweet Heart' is a NZ hynrid with C. paniculata in its make up and I would guess one of the greener species. As your picture shows, it has a nice scrambling habit rather than a climber. I rather think it should be 'Sweet Hart' for Jenny Hart whose garden it came from (I think) but anyone not knowing that would automatically use Heart. It is quite like the x Cartmanii 'Joe' clematis which flooded Europe some time ago.
Mine is also a hybrid but between green flowered species, and in fact is a seedling of a seedling and I'd be hard pressed to name its original parents now. It too is a scrambler, or even quite flat-growing unless held up as this one is.
