I think the best time to move it would be spring however this may affect next years flowers, alternatively you could do it after flowering so it's not out of the question to go for it now. However, you would need to really look after it once it's re-positioned and water it religiously especially in this hot weather.

It has stopped flowering, but shoots are developing at an exceptional rate, its all i can do to keep up with them.It will be half way down the washing line soon, which will not please the wife at all , as she lives and breaths Washing Days, yes Days!! She will wash a tea towel till it dies the death.Square Deal Surfed ,what a way to go .

You could simply prune the clematis our montanas are thugs and have to be pruned seriously.

They are rather vigorous. Ours covers a 12 by 12 Summer House and a 30 foot tall Hawthorn tree.
You can actually move them at any time, though Autumn is probably best. Cut it right back ,now to say about a foot from where it is planted. Soak it for a week, then dig it up with as big a root ball as you can manage and plant it some where else. Keep it well watered after until you get new growth.