Ok, this is proving a very difficult decision. From your replies, it doesn't sound as if clematis are that happy in containers and given the fact that I've found it so difficult to get the right depth of container anyway, and now I'm wondering even if the soil part of the plan is deep enough for clematis.
Mike, your mention of quick thick cover and roses has made me think. For some reason, I didn't think I could get climbing roses to grow in containers (pls forgive my ignorance). I wanted roses in this area in the first place and removed a big old neglected honeysuck bush thinking that there was all soil underneath it, only to find a big slab of concrete for half of the area, hence resorting to a container.
I would really appreciate some recommendations. It is a shady area, may be lucky if it gets a couple of hours sun in the morning but it's a very bright area albeit not a lot of sun. It's an L shape and there are concrete walls/trellis work for them to grow on.
I like what I call a "proper" old fashioned rose shape - not the ones that look like ranunculus or the single petal ones. Ideally I would like deep red, tangerine/peach and possibly one of those gorgeous pale lilacs.
I've found a huge tub at B&Q for £12 - I'm talking probably 70cm wide and 50cm deep which would house 2 of the roses and 1 would go in the soil. I've got wire ready to go on the concrete bits.