you can put any tree in a pot but you usualy have to trim the roots every other spring . I think youd be better with a shrub .

Or plants such as standard fuschia or standard bay. You need to either insulate the pot in the winter or put it in a cold gh, if the roots freeze on any plant it may die

None, really, not if you want something to reach that height. If you got a pot that was 2.5 - 3 feet deep and 18 inches wide, then you might be lucky - you could try Berberis darwinii, or bamboos such as Phyllostachys nigra or P. aurea, which do fine in pots, provided you insulate them well if there's a cold winter.

On advice gleaned from here and elsewhere, I've today bought a half standard Photinia fraseri 'Red Robin', which I'll be planting in a 50cm deep container. It's already almost at the height I need it to be, and by the end of the year, the height will be perfect, and the overlooking window will be completely screened. Thank goodness!

Thanks for all your responses, and happy gardening! :)

your welcome