I'm jealous. We have had no wall, and I've had a small one built, but the
thought of having a go at over 100ft!!!!

Others have provided useful advice....

Alan wrote:
: * how easy is it to train a fig tree along a wall? I've had a free
: standing one before but it was quite in an exposed spot 400ft ASL and
: although it survived it didn't really prosper

Just add that the books recommend digging a hole aprox 75cm cube, lining
with flag stones round sides and at bottom, but leaving suitable holes for
drainage, then filling with builders rubble at bottom and good soil and
compost etc. Plant in this and it should constrain the roots sufficiently.
The builders rubble ensures calcium.

: * how to choose the right "shape" of tree - fans, espaliers, etc.?
: (this is a real mystery to me)

Best not to be perfectionist about it. On a wall its just to ensure even
spread on the wall. More Horizontal stems tend to encourage the growth of
fruiting bud/spurs

: * I've not grown olives before but I've heard rumours that it can be
: done and that with a great deal of TLC & a bit of luck with the weather
: they can even be persuaded to crop - is this true or just wishful
: thinking?

Errr why grow olives. Have you read up on the process that makes them
edible? see

http://www.oliveoilsource.com/olive_recipes_.htm


: * Will the north facing wall be a dead loss for fruit trees? Or is
: there something that will thrive there?

Grow white/redcurrants, and Morello Cherry. Raspberries can cope with some
shade, but fully north facing might be too much.

: * How close to plant the trees together?

depends on the root stock. Send off to various fruit tree nurseries for
catalogues and lists, some have advice on spacing etc.

cheers
Jim