I can tell you how and when to prune 'The President', Ian. It's a group 2 Clem, so it only needs a very light prune to tidy it up in early spring - maybe to keep it the size yoi want it.

I'm surprised that your other climbers are doing so well, if they're in the same sized planters. You can prune them after they've flowered to keep them to the size you want them... What you haven't said is a) how much sun do they get and b) have you been feeding them and on what? Also - how old they are.

I am in no way an expert but I was once told to train clematis horizontally to promote flowering. I also grow about 14 Clematis in planters and pots, I use John Innes 3, replace top couple inches of compost carefully every year, feed regularly, treat for vine weevil (prevention) and they grow and flower very well in a lot of cases better than those growing in the ground.

Not heard that Cestrian, i get plenty of flowers on my clematis when they're trained vertically so i think they must be happy either way :-)

You are probably right Louise1, I was told this years ago when new to gardening and must have had success and took it as being true,it may have been done as a way to cover wider area of trellis. I now grow a lot with obelisks and continue to do that to stop being top heavy.

I think it was probably to encourage the shoots to spread out rather than end up in one tangle, Cestrian. I do try to tie each individual stem in - not always easy though!