Many thanks for your reply, Ken.
I've taken off the bottom and think I see the spindle (a short grey cone-shaped thing, leading through a couple of grey tubes to the black bars that attach to the orange lever on the outside?). But no matter how much I press the bars or jiggle the lever, nothing's moving. I've added a couple of drops of oil where the rod passes through the grey tubes in the hope that might help but am wondering whether a well-aimed tap (to what? in which direction?) might be called for?

sarah, June 2010

Hello Sarah,
If you turn the balance wheel by hand towards you so the teeth go down and at the same time try and move the lever that sometimes works, otherwise as you say a tap with something such as a big pair of scissors to the right and try and move the lever at the same time should shift it, failing that I am stuck.
Ken

drivewithken, June 2010