Hello and welcome Ampthill gardener. I'd never heard of Sorrel Tree but google tells me that it's also called OXYDENDRUM arboreum or sourwood. Burncoose Nurseries (they advertise on GoY) sells them and claims that they are fully hardy to -15C.
http://www.growsonyou.com/shop/product/46877

According to my sources, Sorrel Tree (Oxydendron arboreum) is hardy to -28.8º C. Other than that, I really don't know how well it would do.

We have three Oxydendrum in the garden here, they are perfectly hardy and quite widely available in the UK.

O. arboreum is a really very beautiful small tree or large shrub - ours are all shrub-like in habit. The autumn colour is superb as are the Pieris-like flowers. They do best in a humus-rich, acidic soil and associate well with other ericaceous woody plants.

In their native habitat pretty much all of the Oxydendrons grow in sloughs or swamps, so on a chalky hillside is a no, but in the bottom garden, near a stream or pond, is a go.