It is moved by a variety of things - dabbling birds and mammals (seeds getting attached to feathers, feet etc), mechanically by the movement of soil containing it, and also by detachment of plants through flooding etc. The question I'd like answered is where the U.K. plant came from? Crassual helmsii is Australasian - the New Zealand form (and the type caomes from here) is a small gracile plant, the Australian one is much larger (it even once had a spearate name). Can you send images of what you have over there?