Many thanks to all those who replied to my query.  The job is now
done, thanks mostly to advice from Paul Camps who I also spoke to on
the phone.  After his MMD posting he remembered that his machine had
removable cheeks.  So the answer was quite simple.

The cheeks are not glued in place but held by four fairly large wood
screws each side, into the sides of the cabinet.  The front legs have
simple right-angle steel brackets screwed to the undersides of the
cheeks.  Having removed all these screws the whole key table and
cheeks, with the stack hanging underneath, slides forward.

The only other thing you have to do is drop down two steel control
rods which come up through the right hand end of the table, and release
a similar rod on the left.  Removing the clamping screw releases the
operating arms and allows the rods to drop through into the bottom
section.  You can then slide and lift the whole assembly away from the
piano.

Having done this, the panel holding the exhausters, reservoir, etc.,
can be released and lifted out.  I have now removed the stack from the
key table and put the table plus cheeks back in place for the removal
men to lift by, on Thursday.  The machine dates from about 1911 or 1912
since the list of patents printed on the reservoir stops at 1911.

The suggestions to remove from the bottom may have worked, but in
this case space is very limited in its present location, and the
purpose of the exercise was to lighten the piano to make it easier to
up-end and maneouvre through a narrow doorway and hallway.  Hopefully
this experience will also help anyone else needing to get at one of
these in the future.

Thank goodness for MMD!
John Farmer