I cannot find anywhere any reference to the minimum temperature in C a Clematis Montana Rubens can endure in the winter. Like to hear!!
I've seen C. montana rubens thriving in Providence, RI, around zone 6B.
All montanas died for me last winter. I'm in the warmer part of zone 7.
Tom - it will be fine pretty much anywhere in the UK. Maybe that's why you have found no reference to minimum temps - it's not an issue here. They're just assumed to be hardy. Where are you located? (BTW I just Googled 'C montana rubens minimum temperature' and immediately found a reference giving temps in F and C)
Here is a link that might be useful: C montana v. rubens