If it's only a metre in height that you need, wouldn't it be better and more feasible to grow native herbaceous perennials on the top of the Cornish hedge as is traditional ... I know they'll not be evergreen, but even in winter the dried stems and seedheads willl provide some screening effect (which is usually more important in the summer anyway). They'll not damage the structure of the hedge as evergreen shrubs and trees would do.
My alternative would be bespoke willow fencing, woven in situ ... several examples of it being used to extend the height of a wall on this site http://www.bramptonwillows.co.uk/fences.php
.... different sort of budget of course ... a lot more expensive than some foxglove, red campion, red valerian seeds.