Property immediately at end of garden, advice on improving view?

This is a very approximate idea (Paint and I don't get on too well ) of how to disguise the rectangular lines of the plot and distract the eye from that wall. The principle of having a round lawn is to introduce curves as well as making it much easier to mow! It will also give you a bigger patio space. Another sound design principle is 2/3rds evergreens to 1/3 deciduous plants. There are loads of small to medium shrubs with various foliage colours to choose from.
Personally, I wouldn't plant a deciduous tree. What about Arbutus unedo rubra? Slow growing, evergreen, nice bark, pink flowers and 'strawberry' fruits. It's very attractive all year round.

 

One drawback with Arbutus is all the "strawberries" falling, they make a real mess, but then there will be drawbacks and advantages to all plants and trees.

 

Deciduous trees look beautiful in winter just from their shapes. They change with the seasons, and you can grow Cyclamen under them in their leaf mould, and crocuses.