Standard tree/shrub suggestions please

Hi all

Cracking on with our pseudo-rock garden at the front of our house. 

Its a triangular plot, about 5x2.5metres. the two straight sides are bounded by a brick wall, and I will keep a small border along there. There is a small acer, pittosporum and a blue arrow - and I will likely plant this seasonally. It will soon have a border of lavender in there, a curved border along the longest side which borders our driveway. 

The plot receives a good amount of sun, and gets pretty warm and sheltered due to the brick walls, brick driveway, and back of our house. Essentially full sun about 9am-2pm in summer, at which point it starts to become a bit shaded by our house, but remains warm

I'd like a small centerpiece in the middle, which will be gravel/pebbles. I'm hoping to shove some larger boulders and/or slates in amongst which to plant alpines. The centerpiece specimen would finish it off quite well, I think. We're VERY alkaline (hence the lavender!) and pretty well draining

Any ideas? I think in a ideal world something evergreen or with evergreen interest would be nice, as apart from the blue arrow and pittosporum, the only structural plants in the borders are deciduous acers

Thank you!