If you cut it back now, you will have no flowers next year, and leave the plant vulnerable to frosts over winter. Pruning of these usually takes place in April, as new growth begins, when you remove last year's dead flowerheads (which should be left in place for added protection over winter) and any dead wood.
However, if you need to cut it back to move it, then do so now, but at the risk of its not surviving, and sacrificing next year's flowers.

Thank you Bamboo,its about 3/4ft and I wondered if it would take to it`s new home if I left all the blooms on,it has just got too big where it is and really does need moving.

Bamboo, I wonder if she removed all or most of it's leaves and thinned out some older branches, ?

Annabella, don't know what your weather's like up there, but here its been positively summery today - I'd wait till the weather cools down more, say another 2/3 weeks, and the ground is wet, preferably with more rain forecast, then carefully dig it out, keeping as much of the rootball intact as possible, and without breaking roots, and move it complete, as it is - by the end of October, it'll be dying back for winter anyway. Or you could do it when its completely dormant, after its died back completely for winter, when it won't notice much at all. That way you don't have to cut anything off.