Whoa!! A 90º slope is straight up and down, Crv! Do you have a rise over run ratio, or maybe a better estimate of the slope angle? As it is, it sounds like the old Appalachian joke about the farmer who fell out of his cornfield!

lol, sorry was tired have updated the question! 45 degrees!

Groups of small conifers in different shapes and sizes and colours would work alongside heathers and low growing hardy evergreen shrubs with lots of bulbs in spring?
Acers get windburn very easily, so an exposed site would not suit them so well.

There are alpine gardeners who would kill for a slope like that. Almost any plants sold for rock gardens would thrive in those conditions.

I'd add that flowering cherries won't do - they're surface rooters, and on a slope like that, will be at extreme risk of drying out. By Acers you presumably meant japanese Acers - as Ojibway says, that situation will be far too exposed for them.
A lot depends on whether you're able to actually work on the slope, and whether you'd want to do it frequently, or whether you just want plants that can carry on without the need for regular intervention from you.

cant add much else except welcome to GoY.

Thanks to you all, the garden is large, the house is in the middle of the plot. There is a 45 degree slope to the SE, a 45 degree slope to the SW, level NW and level NE with a steep slope down to a lower NE plot. We have 2 ponds (was 3 but we filled one), a dovecot (desperate for some tlc) and various sheds, a greenhouse (needs repair) a garage and a drive for parking not related to the garage! The drive has 17 steps upto the house.
When I've figured how to load some pics I'll post them, we are in the middle of town but isolated on top of our hill, no neighbours- well down the hill there is a couple of houses!
The hard NE winters are the major issue, as we are high up the summer temps aren't a big issue. We have a lot of mature trees, and lots of laburnham trees which are at least 20 years old.
I like Ojibways idea particularly for the SW Slope as it faces the main road and I have lots of rocks suitable to be recycled on it.
Thanks again, CRV1963

Bamboos, esp spreading ones love slopes. There are varieties that only grow to 50cm or so and you can get ones that will love shade/semi-shade/full sun.

There is one Japanese Maple/Acer that I know that will tolerate full sun and wind and that is 'Orange dream' - as everyone else has said most (almost all) Japanese Maples will not do well in full sun and windy positions (they grow naturally in woodlands where they are well sheltered).

A rockery type garden does sound a good bet.

Some conifers might suffer windburn in that situation so take advice from a reputable nursery re variety. Larch should do, and blackthorn, if you don't mind the thorns.You get lovely white blossom in spring and sloes in autumn. Some of the more vigorous perennial geraniums would make good ground cover, perhaps with foxgloves that would self seed and naturalised daffodils - shorter varieties though because of the wind. Rosa Rugosa will put up with wind and you get lovely big flowers followed by huge hips. - my mother grew it 1000 feet up in Sheffield. Flowering currant should be tough enough and so should forsythia. Hawthorn too would be OK and again you get blossom berries and bird food. Leave one or two privet though perhaps, if you like the smell of the flowers - not everyone does.
Near the bottom of the NW slope you might try some of the smaller rhododendrons if the ground is at all acid.

Thankyou all, I have been given some great ideas. I have a couple of Acers I bought last spring in 6" pots about 12" tall I might try them planted under the shelter of the established trees. I like the idea of bamboos near the top of the slope which will give us some privacy from people walking past looking up the hill. We really want to keep the slopes low maintainence as we have the rest of the garden to work on! I'll post some pics soon. The garden currently looks it's best April/May when the Laburnham are in flower, though we planted a thousand bulbs last autumn so hopefully we'll have a bit of spring interest this year.

We have some photos of the house from the twenties when it was surrounded by borders and beds of daffodils which inspired our first real venture into sorting it out.

I thought it was a mistake--much like I might make, when tired--and 45º is still a pretty steep slope. If there are some areas tending to gully, you might want to plant some strategically placed boulders, with "pools" of gravel behind them, to encourage the water to soak in, rather than run off.