Leylandii have their place - hedges of it tend to be a pain as they get over-large, but a single tree, particularly a golden one, might be just what you want. If you plant it as part of a group of other things, they're good. That said, there are some excellent thuja which might be better scaled for you. Depends how tall you want to go. Both look nice with yellow-variagated hollies and with fatsia japonica which can both get pretty tall. Look to spend at least £20 on each plant (more for the conifer) and you'll get ones big enough that they should romp away and do their job pretty quickly. Phyllostachys (green or gold bamboo) are also great but decent ones will cost £40ish. Look for one that's absolutely stuffed in its pot - it'll put out new stems within weeks of being planted, especially if the soil is moist. This type of bamboo just grows in a steadily increasing clump - but be warned - others can run under the ground and burst up all over the place (even thru patios etc). The benefit of a bamboo would be that it moves in the wind, making the eye tend to focus on IT, rather than what's behind. You could also do a trick where you put some sort of focal point low down and slightly off to one side of the offending wall. This will tend to pull the eye away too, even if its just an architectural plant or colourful pot or something.