which fast growing tree is best for privacy in the garden?

Why not try bamboo?

There are varieties with coloured stems i.e. black (Phyllostachys nigra) and gold (Phyllostachys aurea).
The benefits are that they are evergreen, quick growing and they have a lovely rustling sound in the breeze - they'll also help to deaden sound from outside your garden.

A large plant is not cheap, but being tall (around 8 feet) two or three will fill a space nicely and have an immediate screening effect which, I presume, is what you want.

Thanks so much for the suggestion! I had thought about bamboo, but as the garden already has a few mature trees in it then I think 3 trees would look better, more expensive though I know!

Just not sure which to go for....

If you do choose bamboo be VERY careful which one you get as some are very invasive.

Try not to think about being unable to see the new development, or for the view from it, but instead aim to take your eye off it by planting an attractive perhaps twiggy tree (far less expensive than an equivalent sized bamboo). Suggest, too that deciduous trees are probably more in keeping with the trees you already have. Try prunus cerasifera 'Nigra', pyrus calleryana 'Chanticleer', or a couple of the flowering cherries, or a rowan. All of these will be about the £40 mark from garden centres (8ft plus), or wait two weeks and source them bare-root for considerably less. As A says, bamboo can be a big mistake

For more speed you might consider adding trellis to the top of your wall. You can't see through it from a distance and you can grow some lovely climbers up it next year.

In my opinion bamboo is a nightmare. It is very invasive and will come up through its roots yards away from where it was originally planted and is hard to get rid of. It took me 2 years to finally eradicate a 'Dwarf Bamboo' that someone gave me.

Why not try Leylandii? I know it's got a bad reputation, but if it's kept at a reasonable height it's an ideal barrier for privacy. It grows quick, it's evergreen and you can get it in some lovely shades. I bought some gold coloured Leylandii 3 years ago to replace the privets at the bottom of my garden and they are lovely. I trim them twice a year and I'm very happy with them.

I think you can buy these just about all year round at most garden centers.

As you're lucky enough to have a wall, I'd agree with Steragram and go with trellis and climbers. Hide some mirrors and you'll also improve your outlook by reflecting your garden back at you.

Thank you a much for your suggestions, I can't use a trellis unfortunately as there are comings on top of the wall, also it is already quite high so wouldn't look right. The development has been built on a higher level to us so that's why they will have a view into our garden.

I think I will go and see what bareroot trees are available, but I like the flowering cherries idea.

Thanks again for your help!