Thank you all for the ideas. I'm off to look them all up. Thanks for the mixed native hedge idea Rhubarb garden. We get lots of wildlife in the garden so something like that to encourage them to stay is a great idea.
How about a mixed native hedge? I know you said you'd prefer one type of plant for ease of maintenance, but a native hedge is treated all the same. You just cut it back once a year in early spring, it's tough as old boots, great for wildlife, would blend in perfectly adjacent to a field of sheep and is very attractive. Ashridge Trees supply quality hedging mixes and their website has great advice and info on planting and care. This one is a thornless mix and includes lots of flowering things so would be very pretty.
Bamboo - I would be very careful with this, although if you mow around it I think you could probably keep it under control. Privet - would need cutting 2x a year really. A yew hedge would only need doing once a year - would take some years to get to the right height though! Pleached hornbeam or lime would look stunning but would need pruning....
I was wondering about rhododendrons / Azaleas - they often like clay, if you got the right ones they would be about the right height, some of the rhododendrons keep their leaves, and they of course have nice flowers in late spring.
Don't get a holm oak - they grow huge and kill off everything underneath.
Hazels might be quite nice - fairly quick growing, and shouldn't be too tall - you could even have a few copper ones too. If you don't have too many squirrels you might even get nuts.
How about bamboo? It can go mad with runner shoots but there are ways of stopping that (boxing in roots for eg). There are varieties of bamboo that get very tall, but equally you can get bushy clumpy kinds that go up to about 8 feet that are easy to prune.
Other options - small trees? Hawthorne or mountain ash for eg? Holm oak is evergreen and relatively slow growing but it will eventually be a large tree.
How big is the area you need to cover?
Another option could be standard privet (ie privet hedge grown in a tree shape) which would provide screening at eye level and above, ideal for use with a fence to increase height of screening. Needs cutting once a year though.
Mistlethrush, you're spot on. We need something that will act like a hedge but doesn't need trimming constantly to keep the height down. So something that grows to between 6 feet and 8 feet would probably be about right - we can plant higher up the slope if it's a naturally shorter plant/shrub. As far as whether it's one thing or a combination, I think, knowing how much time we (don't) have to prune/maintain too many plants, we'd be better off with one type.
I'm not a natural gardener - I tend to be a natural destroyer! Give me a patch of weeds or an out of control shrub and a few hours and I'll rip it out/cut it down or prune it back, but maintaining things tend to make me break out in a sweat as I have no idea what I'm doing! So something that needs a trim a couple of times a year and will then just get on with growing, works well for me.
I'm not worried if it's an evergreen, though I'm just as happy if it is. We don't tend to use the bottom garden much in the winter - so we need the screening more in the spring/summer when we're down there a lot and we use the tree seating area.
Our back garden is in three 'terraces'. The final one slopes down about eight feet to a flat area which adjoins a neighbouring field. The slope on one side doglegs round so our neighbour's back fence is at the top of it. There,s a small tree at the point where our side fence joins their back fence and we have a tree seat there. Our old neighbour had a thick hedge up at the back of their garden as they had dogs and there are sheep in the adjoining field.
They've now moved and the new neighbours have removed the hedge as they wanted to enjoy the view of the field and sheep. Which is fine, but they didn't realise our garden doglegs round behind theirs (why people don't check land registry things I don't know...). Which means, when they are on their patio, they can see right down into our garden. And, of course,when we're sitting at the tree seat, we can see right into their house...
So, I need a shrub that I can put on the slope to effectively give our lower garden some privacy BUT still give our neighbours some view across the fields. We need to cover about forty feet of fence. If I can get something fragranced or colourful it'd be a double delight as we could enjoy it sitting under the tree.
The garden faces west but we do have a few trees making the area light shade. The slope is quite angular at that point so something that won't mean me scaling Everest every few months to prune it would be good plus we have a six year old so too spiky wouldn't be great. We have clay soil but, as the trees have provided a good mulch every year, the soil's pretty good in that area and we have a little natural stream running off the fields coming through so it's very rarely drought dry.
Any ideas would be great. The rear fence of the neighbours garden is theirs so we can't really improve the height or type.