Buying large plants from garden centres costs real money - and so do the large containers needed for them.

You could ask if anyone wants to get rid of large container plants on your local Freecycle. If it's somewhere windy you'll need to secure the plants - tie them, or better still, tie the containers, onto the railings?

I think what you want is going to be very difficult to achieve - large plants in containers are unstable and need A LOT of watering and feeding. And plants don't stand still. The tree that is 6' high this year will be 8' high next year and 10' high the year after - assuming it hasn't blown over and taken out your or your upstairs neighbours' windows with it. That's why people grow dwarf conifers in containers.

Your other challenge as you live near the sea is going to be salt spray - very few trees take kindly to this. A 26 inch wide ledge is not really wide enough to house the very large container you will need for a 6 foot tree. Beattie is also correct to point out the tree will want to keep on growing.

What about Bamboo? As the others say wind and salt spray would be bad for a lot of trees. Bamboos would allow the wind to pass through whilst making a screen.

I've bought the type of plants you're talking about Katya.

As Beattie says, they do cost a lot but if your disposable income allows for it then try your nearest local nursery because you may well find that they have suppliers, that's what i did.

I paid £35 per shrub for plants which were about 5 feet high by the same across. When you buy more you often get that price discounted aswell.
It's definately worth buying like this when you don't have the time to wait for the plant to make the growth.
I needed hedging plants, and fast, because privacy was a very big issue and this worked brilliantly well for me.

Google 'mature shrub suppliers' - theres the Bigger Plant Company and Old Hall do them too.

Thanks to everyone. I followed Bamboo's leads and have had some good advice from the Bigger Plant Company though logistically it could be expensive as we are in the South East. They advised looking for something nearer home but I haven't yet been able to find anything significantly closer that offers a similar service. The plants they suggested - standard hollies or pleached hornbeam - come in manageable containers, and I assume that these would restrict root growth and therefore the eventual size of the plants. (My husband is very taken with the idea of the holly.) A taller plant wouldn't bother me as such, but the important point is to keep the foliage at the level of the window, and not have it take off skywards if the stems continued to grow. We won't be doing anything immediately as we're about to have the house painted, but I'll let you know what happens.

These people are in West Sussex.
http://www.bigplantnursery.co.uk/index.html

And this one's in Essex
http://www.kingco.co.uk/

If they don't have what you want, they may know someone who does.

There used to be one in town somewhere, Chelsea, or somewhere like that, can't remember what it was called - I'll do a bit of research, might be that its gone now though.

Thanks, Beattie. The Big Plant Nursery are helpful and their delivery charge is a reasonable £50, but they don't seem to have anything the right height just now (either too tall or too short). Meanwhile I have another question. Today we found two rather tall standard bays locally, which are selling at half price (£99 each). They're not quite a pair as one is perceptibly taller than the other, but as they're in quite small plastic pots at the moment I think I could level them out by planting one higher in the pot when I replant them in deeper containers. My questions are, firstly, once the stems have grown, do they stay at that height rather than rocketing on upwards? Obviously we want the foliage to remain at the same level. Secondly, will the foliage continue to grow upwards or will it too remain at the same height? (I know the heads will have to be trimmed, but perhaps only to keep them tidy.)And thirdly, can the stem still produce side shoots, especially nearer the top (which would enable us to sort out the difference in the height of the foliage)? I realise that I know nothing about standard clipped trees in general, but I assume they come as the finished article - if not, how can designers and architects work with them? However, I did buy a small bay for culinary purposes a few years ago, which I then neglected as we were away, and when we came back it had turned into a bush. It's now part of our hedge.

Katya I'm not going to comment on the growing habits of bay trees, I've never grown one. But, when you plant them you mustn't plant deeper than they are already planted - this goes for any tree.