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a) Are we right in going for the Western Cedar?"
Yes, good choice. Quick, but not as fast as Leylandii. Trims to a nice tight / smart hedge. Can be cut back to old wood (not as well as others, such as Yew, but Leylandii will not resprout from old wood at all) - but better never to miss an annual cut (and you may want to cut it twice a year, once established, depends how "smart" you want it to look)
Personally I prefer the "Atrovirens" cultivar - they will be more consistent than just any old "Thuja plicata" - but it probably doesn't make a lot of difference.
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b) We want a full and thick (I want a blackout!) hedge from 0-6/7ft - how far apart should we plant them?"
2' should be fine. Single row. Depends a bit on how tall the plants are that you are putting in. The taller they are the further apart you can get away with.
Don't plant bare-rooted now, its too late. Either wait until next Autumn, or plant container-grown plants. If you have a hosepipe ban where you are then better to wait until Autumn, or install some drip irrigation (and pressure reducing valve and timer - or whatever is "exempt" by your Water Company). If its only half a dozen plants then obviously you'll be able to water them with a can
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c) Should we leave the ground alone for a while between taking the old out and putting the new in? If so how long?"
No, you can plant straight away, but you need to get the old stumps out. I used a JCB to replace mine!
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d) Should we do anything to the soil inbetween the old and new change over?"
Incorporate some well rotted manure, or mushroom compost (depends how long the row is, if its short them some bags from the local garden centre will do, but for a long row it would be cheaper to get something in bulk, or a local farmer / horse riding stables to deliver if that's an option)
Dig a trench, use a string as a "line", cut a piece of stick to the desired planting distance, and use that as a "spacer" between each plant. Put some general "Growmore" fertilizer around them after planting (but not too close to the trunk).
It is
critical that the plants are not planted too deep - that's the cause of most newly planted trees sying. Plant at the same depth as the plant was before - i.e. if from a pot then plant so that the soil level in the pot is the same as the finished ground; if bare-root then you should be able to see a "soil mark" on the trunk, plant to that depth. If in doubt plant slightly-shallow rather than slightly-deep.
You may want to use mycorrhizal fungi which are suppose to promote faster root growth, and get the plants established more quickly. These MUST be in contact with the roots - so sprinkle on the root ball after you take it out of the pot and be careful it doesn't get dislodged when you put soil around. (For bare root dunk the roots in a bucket, then sprinkle on and it will "stick" to the roots).
I install leaky-hose along new hedges, then a mulch (well rotted manure), then weed suppressing membrane (the woven sort) and then something on top so the plastic isn't visible - bark / wood chippings. That will keep the moisture in.
Put a shallow container under the leaky hose and run it until it has 1/2" to 1" water in it. Do that once a week during the Summer, and twice a week if the weather gets really hot and dry. If watering by hand give the 5-10 litres each if the plants are 2' tall or less, 10-20 litres if you are buying in taller plants. Watering well, and then waiting, is much better than watering "a little bit every day"
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e) Is there a 'management' bible on how and when to trim the trees to get the blackout effect that we want??"
Hedge will probably want to be 3' to 4' thick. Once it starts to get to within a foot of that start "nipping" the shoots that are jutting out over that imaginary line, this will cause it to branch and that will thicken it up - plus the bits in between the plants will start to fill out to match the widest parts.
Let the top grow 1' higher than your finished height, then cut the tops to 6" below the finished height, then cut it where you want it when it has grown back.
When the sides are over the intended "line" then trim with hedge cutter. Ideally have a "batter" - thicker at the bottom, tapering towards the top, as it lets the light get to the lower parts of the hedge, although most people cut their hedges with vertical sides