Welcome.
I've added this question to GoYpedia Climbing Plants ..
you may find some ideas there.
I hope this helps. :o)

Perhaps a blue clematis?

I cant answer ypur question but hello and welcome to goy x .

Welcome to GoY, Marion! Remember that a vine will only grow about 4 times as big as the tub it's growing in--i.e., a 2 foot tub will support an 8 foot vine. If you are thinking of two vines in the same tub, halve the size of each vine.

i have one of these jasmines in a large tub with a clematis Barbara Jackman growing through it. Works well most years but does need regular watering of course.

Thanks for your help everyone, Penny is the jasmine evergreen and also the clematis. It says on the description that it is so I was a bit concerned if I put a summer flowering clematis with it I might have trouble detangling it when it needs to be cut down, how do you manage it.

Well, Marion, my jasmine clotted cream is not evergreen. I bought it from T&M a few years ago. It took 2 or 3 years to really establish itself and bloom properly. I wonder whether yours is different? I don't worry too much about pruning clematises which are growing up through shrubs--it's too fiddly and time consuming--I just tidy up both plants and let them get on with it! Hope this helps.