Box plants are a nice shape and are very tolerant of shade, for example.
Or baby phormiums have attractively coloured leaves, and are a nice
striking pointy sort of shape (though they will outgrow your pots in
time).

Thanks Victoria
I've heard of box (will wonders never cease?!) but not phormiums - I assume
they are readily available through most garden centres? I also assume I'd
be buying plants as opposed to seeds or bulbs or something?
Is now the right time to put these sort of things in or do you think I
should wait until a few weeks before I put the house on the market?