Perhaps have a look at Erigeron karvinskianus, trailing nasturtiums, hardy geraniums eg Bill Wallis is a trailing one, aubretia and see if you like any of these.

I should have said - south-facing flowerbed but in shade after midday because of shadow of the house...

white candytuft?

Phlox subulata and douglasii.

hey - how quick are all of you?!! thanx all, will check out these plants, have already planted aubretia and a smaller campanula (not as big as this one) in similar bed but more sunny situation, both growing too slow for my liking!! cheers peeps for your info :-) j

That camapanula is a bit of a thug and will quickly swamp your hostas if you give it half a chance! Geranium sanguinium makes a lovely front of border plant. Takes a year or two to get going, but develops into a lovely neat mound about 4 inches high and flowers all summer.
One of the low growing blue perennial geraniums would look lovely under your sambucus niger - not always easy to find, you might need to use mail order. Johnson's blue is quite easy to find and beautiful but might spread a bit wider than you want and tends to have long rests in between flushes of flowers. I would search for something a bit more compact.
Try not to be in a hurry, because the ones that grow quickly (like your campanula) don't always stop when you want them to, and in a year or two you will be wanting to throw them out!

Another suggestion is the hardy fuchsia Lena. It would overhang your wall and flower continuously from some time in July until the frosts and it is beautiful

wow - plenty to think about now! thanks steragram for your advice, j :-)