Depends whether it remains sunless all year round, or gets some sun at certain times. On the assumption it remains largely sunless, the only 'tall and willowy' plant I can think of is one of the shade tolerant bamboos, such as Fargesia, or Sasa palmata. Otherwise, Fatsia japonica, Elaeagnus and possibly Pieris forrestii varieties such as Forest Flame (but not Pieris japonica varieties). The latter would need ericaceous compost.
To reach any height, these plants would all need (eventually) a pot 18 inches deep by the same wide, more if possible.

THANK YOU, I SHALL DO MY HOMEWORK ON THE ONES YOU HAVE MENTIONED.

How tall are you thinking? I have a flowering dogwood planted in a shady border that will only get to eight feet tall and no more than half that across - cornus kousa 'Cherokee.' Although it is not evergreen, it has an attractive outline for winter interest.
Check out www.junker.co.uk - they offer a huge range of cornus and other shrubs and Karan, the owner, is always very helpful. Although the prices look a bit steep, you get excellent plants of a reasonable size and I've never had cause to complain with anything I've bought from them.

CHEERS, ANDREW, I WILL INVESTIGATE 'JUNKER'

Is it true that Garrya elliptica will grow happily without much sun???

Yes, mine is growing on a shady east-facing wall and flowers every year (except last year in the very cold winter)

I had one trained flat on a north facing fence and it grew and flowered very well. It was just outside the window and when the light shone on it at night the catkins looked fantastic. Was sorry to leave it behind.