What would grow tall here?

So this is at the back of our garden in the top corner, the wall (soon to be cladded to look nicer!) and fence with an overlooking house behind, currently partly shielded by budhleia trees growing from the top of the wall. We want some privacy shielding from the part of the house you can see in the photo, the height of the wall and fence combined is approx 12ft so if we planted anything in the ground even established would take an age to reach a height to provide the required privacy so ideally we want something to grow from the top of the wall like the budhleia is. The main issue here is the gap between wall and the fence is only about 10inches, obviously the budhleia has grown in that space, but maybe pre fence we think, so should we just plant more of that? Would anything else grow here in that space or could go all the way along if we got rid of the budhleia which has got very woody and overhanging and looks pretty awful when the leaves aren't on it, would anything else grow from this small planting area tall enough (fence 6ft tall then up to where budhleia is now) to provide a privacy screen?
South facing and gets plenty of sun
@Plantminded you were a great help on my previous post in the 'Trees' forum, any advice on this?
Any advice greatly appreciated! Thanks in advance

 

I did think that gap needed attention in your previous post @Bramble400 . Does the soil behind the wall go the full depth to the ground and is it open to the ground at the bottom to allow drainage? I think you'll struggle to find anything that grows tall in that limited space, apart from Buddleja!

An easier option would be a tree in the ground in that corner. I'd go for a Sorbus, which although deciduous won't get too big. Sorbus vilmorinii is a nice one, but you'll have to be patient!

Sorbus vilmorinii | Vilmorins Rowan Tree For Sale | Ornamental Trees

 

I've had a rethink and edited my post above @Bramble400 .

 

@Plantminded thanks, is there a specific type of budhleia that would be best?

 

Is that window only a problem looking onto the spot you are stood or do you want it to be blocked from a lesser angle? That might not make sense but you can block angles like you have in the picture by planting a smaller tree closer to where the picture was taken, rather than on the wall. If you need the height on the wall as you are overlooked higher up, like on another floor (say two second storey windows facing each other) it won't work.

 

Unfortunately it looks like Buddleja might be problematic in future due to an aphid causing leaf and flower damage, as detailed in a current thread here. The most common one grown in gardens is Buddleja davidii, available in several colours. I’m reluctant to recommend it though until the current problem is resolved. Another option for you, if you have a sitting area in the back is to lower it and plant a hedge around it for seclusion or use decorative steel panels as a screen or a pergola planted with climbers.

 

All making sense @Plantminded thank you.