Posted: 04/04/2015 at 11:14
Digging under that will be a sod of a job. You know that, right?
It'll be a mess of roots. You may find it easier to build a low wall (bricks, turfs or a rockery) and fill behind it than to dig planting holes.
Plant things that are small and let them grow bigger rather than trying to dig out a 14-inch cube of soil and roots from under the hedge. You'll do less damage to the hedge's own roots in the process that way.
Once they're in, your new plants will, as you guessed, need some help competing with the hedge. Where I put in a seedling tree to fill a gap in the privet hedge here, I gave it an advantage by making a big hole in the hedge's roots on purpose so it wouldn't have to compete for a while. It's doing alright. In your case, I'd suggest feeding and watering the new plants often and in small amounts to give them a boost. That got a hydrangea going right next to a sycamore. It's got ambitions to climb the thing now.
As for what will grow there, either something low and colourful to brighten it up or a climber that can scramble up through the hedge and cover the face of it in flowers.
My usual approach to the question is a website with lots of filter options to reduce their thousands of plants to a few suitable ones. For "sunlit, south-facing, bottom of a hedge, climbers," their options are Virginia creeper, ivy and a clematis that only grows 50cm high. Failed.
More luck with "shrubs": 52 results. Up to you which you like. I'm always inclined to add "low maintenance" to that, and in your case the 19 drought-tolerant and non-poisonous ones might be a better shortlist. Heh. VERY short list. Bunch o' Vinca, bunch o' Berberis. Berberis are thorny monsters that make pyracantha look cuddly, so you probably don't want them. That leaves a lot of little Vinca.
Another search: neither climber nor shrub but rose! I don't have any of those, but if they're anything like mine, they'll be all over that hedge in a few years and smell fantastic in summer. Pink and white options there. Maybe atropurpurea goes with veilchenblau and Gertrude Jekyll goes well with Wedding Day or Albéric Barbier, or maybe you prefer contrasting colours.
5m, 7m and 8m heights on those white roses. Not in the first few years, probably, but ... yeah, be aware they'll get all the way up that hedge and pruning and training them will require ladder access.