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I'm interested in buying a climbing Jasmine.

I've seen a 'winter' one in Morrisons which claims to be deciduous, yet is clearly already in flower.

Is this normal?

How can they flower without leaves?

They have leaves through most of the year but flower now...
they aren't climbers but have very lax stems and can be trained/tied in...  :)

Even the name means flowering without leaves, Jasminium nudiflorum. It flowers without leaves so that the flowers are obvious to winter flying moths which pollenate them.
We have a fair number of them, since any stem which touches the ground will root from every leaf joint.