Erm, I think this might just be one wish that's impossible to fulfil. The problem is growing in a pot, particularly in colder regions of the country. First, the pot restricts the growth of any plant you put in it, specially a climber; second, the pot is vulnerable to freezing in winter, and if the compost inside the pot freezes for longer than 3 days, your plant's a goner.
If you want to try anyway, choose the largest pot you can manage - if its a minimum of 2 feet deep and 18 inches wide, bigger if you can, that's more growth room and less risk of freezing. The other restriction is the plant itself - sounds like a very shady situation, so your choices are restricted - Hydrangea anomala is one, Hedera helix varieties, or Fatshedera lizei might survive a winter there - this one has lax stems which you'll need to tie onto the trellis, gets only about 4 feet high, has a variegated version too.

my first thought was Hydrangea petiolaris but it would need a really big pot.

Ahh I was hoping it wouldn't be an impossible task. I should say it does get some sun but not a lot and it's not a solid wall (is some weird block stuff). Last year I grew my potatoes in sacks in front of it!

Hydrangea anomala and petiolaris are one and the same - anomala's just its new name. If the area gets some some sun, you could try Passiflora caerulea, but not sure whether it'd come through the winter in a pot.

Wouldn't the hydrangea get much too big for that area though? A mature one will cover a large house frontage.

Cotoneaster horizontalis? Not evergreen but it makes attractive herring- bone branches. It might not flower and berry very prolifically in that situation but it would be easy to keep flat against the wall in an attractive fan shape and still attractive when it sheds it's leaves.

Steragram - it wouldn't get too big because it can't, its contained in a pot...

AH, yes. Poor thing, a bonsai!