Thanks everyone, I think I'll just stick with the bluebells then!
to each of your specific points:

Phil Gurr wrote:
Do you mean Scottish bluebells (hairbells) or English bluebells (wild
hyacinths)?
I'm thinking of English Bluebells

Sacha wrote:

By 'ash' do you mean Fraxinus which grows to about 100' and is a forest
tree, or Sorbus acuparia, the mountain ash or rowan? You could grow
things like Vinca or daffs and cyclamen under either but a Fraxinus
might give you and your wall considerable problems.
I'm fairly certain it's a Fraxinus - it's bl**dy enormous! I think the
wall and the tree have had plenty time to get acquainted - they both
pre-date me.

Mark McIntyre wrote:

By the way, remember that wild bluebells are protected and its an
offence to dig them up.
I'm buying them off ebay - the seller says they're cultivated, so I'm
offloading the responsibility on them

Thanks to all of you again, you're a helpful bunch.