Hello MMD.  I have received at least half a dozen responses to my
warped key enquiry, for which I am appropriately grateful.  Here is
what I did.

I had a 'phone call from Rick Alabaster, a restorer I know in Melbourne
(which is about a one-hour flight from Hobart).  He suggested I use the
steam from a cappuccino maker.  This presented some difficulties,
because we don't have one, but during our conversation I remembered
that I do have a steam gun (a Sunbeam 1000 watt Eco Jet).

I made a jig out of a piece of chipboard with three dowels inserted
into it so that they stuck up like pegs.  They held the first key in
the final shape I needed.  I removed the key and gave the rear end
seven or eight minutes of steaming.  I kept the steam away from the
balance rail button, the capstan at the end, and from my fingers.

When I figured the key had received enough steam (well, I guessed that,
actually), I put the key onto the chipboard, between the pegs, and
clamped it down at both ends, to guard against warping.  I didn't touch
it until it was dried out.  It seems to have worked well -- the capstan
now sits properly under the sticker, and the key does not rub against
its neighbours.

Regards from John Phillips in Hobart, Tasmania