Im looking for ideas for a small evergreen tree to hide the nightmare housing development which has sprung up in what was once a wildlife haven behind my house. It needs to be smallish, max 15-20ft & fast growing. I have a few ideas but wondered what wo

The only one of the above I have personal experience of is the Crinodendron which is a shrub rather than a tree and, in 15 years stands about 7 foot in our garden.

I'll probably be shot down by everyone else for this idea, but I'd suggest you consider a bamboo. I know they can be horribly invasive but they will grow quickly into a thick clump, and judging from the wildlife that shelters in our clump during the winter, it would be good for that too. Even the tallest don't go much beyond 15 feet. And you can harvest surplus canes to use in the garden.

Great idea, Bertie - quite a few of the phylostachis are clump forming and not invasive.

Bamboo is a good idea - but suggest an impermeable barrier placed 18 inches to 2 feet deep vertically into the planting area beyond which you do not wish it to go - even the clump forming ones now have taken to running in this country, due to global warming. You can use something like vinyl kitchen flooring as a barrier, but don't try weed suppressing fabric or polythene - the roots will punch through it and spread anyway. Check the eventual height of the one you choose - black bamboo (phyllostachys? spelling?)) reaches about 12 feet, others are taller or shorter.
The cytisus you mention would be fine, but they're not long lived plants - the Crinodrendron takes time to grow large enough and is best against a wall or fence for shelter.

Laurel, although not to everybody's taste, grows fast, has fresh-looking shiny leaves and provides shelter and nesting possibilities for the birds, who have just lost their habitat. If your garden is big enough for another tree as well, a rowan would provide food for them and look lovely for at least 3 seasons of the year, softening the brick and concrete jungle beyond.