Rachel Sullivan wrote in
:

How shady? Are we talking coal hole here, or fairly light but not
direct sun?

Oh.....B-)
Its a fairly shady spot....No direct sun after about 9am ....

And how good drainage? Sahara dry, or retentive but not waterlogged?

It will hold water about 3" down....But the top will dry off fairly
quickly...

If it's dry, dig some decent stuff in there & make sure you keep a
clematis well watered (especially when it's getting established).

Yup.....

You could try a montana, (they aren't desperately expensive, so
wouldn't hurt too much if it failed) but the scented ones do best &
the fragrance is more obvious if they get some sunshine.

Well....Its not going to get much sunshine....
Only very early....
It will get more if it grows over the height of the outhouse....

There are lot of clematis that want shady/dappled shade though, generally
the pale varieties that bleach out in the sun. Nelly Moser for instance,
Silver Moon & the (one of) the new Chelsea launches, 'Paradise Queen'
- very Nellyish but with pearly-white flowers in the autumn. Lots
more - think pale delicate colours.

I'll keep an eye out at the garden centre.....Then look to see if any of my
neighbours has one! B-)