Going about business today I naturally veered into a garden center to buy a yellow Hibiscus with red throat, but got this beauty instead, it's called a Newbiscus Mauvelous.
The flowers are 26cm (10in) across. It is hardy but I don't think it will be evergreen (although I was assured it was) but it's not an Hibiscus rosa-sinensis.
According to the supplier's website, the Newbiscus come in four varieties, from white to deep red.
For more information about the Newbiscus, click here.