Thanks for the replies. I've also found some info on the RHS site.
There's a brief description of a 'model fruit garden' at Wisley which
apparently has pears and apples grown as cordons on the east and west
fences. It doesn't say exactly what fruit is growing where. I would
guess the west-facing fence would be warmer and more sheltered and so
is a better place for pears, but I guess it must be possible to grow
at least the apples facing east.