How big do you want it to be, and what do you regard as interesting?
There are lots of things that will be happy even on alkaline clay.

it's not so much the clay that is the problem, more that we have become
paranoid about "roots". I was looking for (a) a tree (say about 12 - 24
foot) which woudl give some blossom and maybe berries, also some evergreen
shrubs which we can plant around the overflows/vents etc to hide them. I
know to avoid hydrandras but am unsure about other shrubs.

--
Hayley
(gardening on well drained, alkaline clay in Somerset)