I'm afraid 'fast growing' and 'about 6 feet' is not possible! Fast growing plants are usually, eventually, very large, whereas those that only get to about 6/8 feet are somewhat slower.

Buddleia davidii are rapid growers, but they need pruning right back to within an inch of old wood each year in March - even so, they will put on about 8 feet of growth during spring/summer after pruning.. Pyracantha (if you can bear the horrible thorns) or Mahonia japonica varieties grow at a reasonable rate, otherwise, what have you planted already that's taller? And how deep is the border from front to back? And are we talking part sun part shade, or deep shade, or full sun?

The border is about 5ft deep, I have planted a couple of deutzias, they are quite slow, we visited an open garden yesterday with lots of height at the back but maybe a lot of them were perennials, sorry I can’t tell you any names ! I’d like to plant in September ready for next year.

And what's the situation re sunlight?

We bought a Passion flower plant from our charity-run garden nursery last year, it was about 2-3 feet high, it’s now about 30 feet - I’ve trained it along our fence, it’s flowered so we’ll this year, followed by loads of green ‘fruit’ which have now ripened to a beautiful orange.....I’ve trained it and snipped it, tied it in to train it and it’s beautiful. Best £5 we’ve ever spent! I’ll try to post some photos!

Fast growing to six feet, evergreen, flowers all year - gorse!

Some clematises grow rather fast and some are evergreen, However I don't think they stop at 6 feet but you can cut them back.
You'll also have to put something to support them like mesh or something.

It’s a sunny border, really struggled this year, sometimes had to water twice a day, I’ve got garden envy from visiting a couple of open gardens recently !

Andrew - thanks for the giggle...

Sarah I think if there were plants like that they would soon sell out! You probably need to choose between evergreen and long flowering. Garrya eliptica is evergreen and has wonderful long tassels in early spring.(variety James Roof has extra long ones but they are all lovely) With care you could train it as a wall shrub without support or preferably use some horizontal wires.It looks wonderful somewhere where artificial light from the house windows can shine on it on a dark evening. Had you thought of "splitting the duty"?ie having one evergreen and one flowerer eg clematis - have a look at Hywel's recent blog for those. You can train those at the height you choose. Training on a fence leaves you more border space to play with than free standing shrubs.

Open gardens often have what are known as perennial borders - that means most of the plants disappear in the winter. Shame you didn't take photos, really, but if it had lots of taller flowering plants at the back, that might have been what you were viewing.

I don't know how long the border is, so here's a few suggestions for larger shrubs, but before buying always check the eventual height and especially the spread, so you can work out how much room they need before deciding. And bear in the mind that the spread, or width, means all round, so if it says 6 feet spread, that means 3 feet from the central trunk or stem all the way round, including the back, so it would need to be planted 2.5 to 3 feet from the fence.

Buddleia davidii for a sunny border, behind other plants, is great, because although its spread may seem enormous, it only spreads at the top - the central trunk or stem doesn't take up much room because of its growth habit. My favourite is B. 'Black Knight', but there are loads to choose from - just make sure you don't choose a dwarf variety by mistake. Berberis darwinii is a nice shrub, evergreen with orangey yellow flowers in May, gets 8 feet, but it is prickly. Photinia 'Pink Marble' is an evergreen shrub that gets like a small tree if left unpruned, but it responds well to pruning. Ceanothus such as 'Edinburgh', also evergreen, eventually get to 9/10 feet, with blue flowers in May; there are lots of varieties, so just make sure you check the labels for height and spread carefully, as well as checking its evergreen - some aren't. Garrya elliptica is a lovely shrub, as said, can be trained flat against a wall or fence, but it does prefer partial shade.

Otherwise, for deciduous taller shrubs, Cornus alba sibirica variegata (don't get Elegantissima instead, it gets enormous) makes about 6 feet with variegated green and white leaves and red stems. Physocarpus 'Diablo' or 'Lady in Red' are worth considering - both have almost black or red foliage respectively, and produce pink flowers in rounded clusters. There are a number of Berberis varieties with, variously, reddish purple or yellow or variegated purple/pink leaves - these differ in height and spread depending on variety, and all are prickly. For deep reddish purple leaves, Cotinus coggrygia 'royal purple' is great - but it does eventually get quite large in spread, although it can be pruned back.

And a word about 'flowering' shrubs. Bear in mind the flowers will likely only be present for 2 or 3 weeks out of every year, so what the shrub looks like the rest of the time is, I think, more important. You can create interest by using shrubs with different size, shape and colour leaves, as well as considering their flowers.

Thanks so much for all your advice, much appreciated.

Sarah, I've read all the great advice above and just wanted to add...have a look at Clematis 'Avalanche'. Its evergreen, the leaves are very pretty indeed and it has gorgeous creamy white blooms in late spring. I used to grow it on a sunny trellis and it grew fast but not too tall. It takes a prune as well. worth a look for covering a sunny fence. :)