Solved Clematis ID

This clematis has been growing over my grandma's front gate for as long as I can remember. It usually blooms spectacularly but is decidedly lackluster this year. We're hoping that IDing it might help us look after it a little better and return it to it's former glory - any ideas? My first thought was Armandii but the blossoms are very consistantly four-petalled rather than five, which is what I thought armandii was.

 

Thanks @Marley Farley - my nana thinks Montana sounds familiar so that's likely it