I liked the advice on Tee Gee's page about growing uprights further down, so they grow up and meet the trailing plants - I've never tried that so I'll give it a go this year.
In terms of buying in plants, it is a good idea sometimes, because some of the beautiful ones are quite fiddly to grow and the professionals do them cheaply. I buy lobelia and petunias. They're so easy to keep, have a long season and are cheap from B&Q and places like that.
I've had bad experiences with buying 'plug plants' online - when they arrived they were only seedlings. If you buy from B&Q or Homebase or a garden centre, you don't pay postage and you can see what you're getting.
Of course I'm not implying that Suttons or J Parker are not reliable - I've never bought plants from them.