"quick to develop and dense enough to be opaque when fairly slender. "

Hawthorn, though it is not truly evergreen (leaves can stay on till the following year)

You can't get much slenderer than trellis with evergreens if you can't think of suitable hedging plants.Most hedges get thicker with age after all- can't think of any that stay slender except for fastigiate berberis such as Helmond Pillar but it would cost a bomb to plant up 5m with them.. Incidentally I've never ever seen hawthorn with any sort of leaves in winter - you must have tough ones up there in Fife Kildermorei!

If it doesn't have to be evergreen a row of Queen Elizabeth roses makes a nice hedge

Look at cotoneaster simondsii or franchettii

Thanks for prompting me Steragram! I meant hornbeam - you can have a very thin hedge with hornbeam and it has Beech like properties.

I thought it was funny! We have beech and it looks good all the year round. But at around 20 years old it has got about 4 feet wide in summer, about three just after clipping. It has already made the drive a bit narrower as it was planted a bit too close to the edge. Not sure how you'd keep it slender, if that's an issue. Hornbeam might have been a better choice if it can be kept slender, if our predecessors had known about it!

You can make quite a slender hedge with hawthorn too, if careful about the trimming. I have it in my back garden in a mixed hedge with pyracantha and escallonia - I now regret planting the latter due to black spot but at least it is thick and healthy at this time of year so I'm leaving it in situ for the time being and the escallonia is also hardest to keep narrow. Hornbeam sounds nice as it has such fresh looking pleated leaves in spring.