The main rooms on the ground floor are about 25ft square. The ceilings are
supported by 12" x 12" beams in a + arrangement. The cross is central to
each room and formed by a single timber span with the 2 'arms' double
morticed/tenoned to that single span. Joists of 9"x6" are morticed/tenoned
to the 2 arms at about 18" centres. A very substantial structure.
In one room only the single span is bowed downwards by about 3" and this is
immediately noticeable. Equally worrying is that the tenons of the 'arms'
have pulled out by at least an inch on each side.

If I understand this correctly the construction method has created a
major weakness right at the point on the main beam where it needs to be
its strongest. Old timber is often bowed but that would have been a
gradual process when the structure was relatively young. A sudden
movement in old age suggests to me that the main beam has failed and is
now being held up by the floor above and the minor joists as much as by
any remaining stiffness of its own. If that is so you need to put in
Acro prop (or similar) to prevent the next stage of collapse. Don't
forget to spread the load if the lower floor is suspended or in any way
suspect.

If you have any heavy furniture above shift it at least to the edges of
the room but not until you have propped the beam.

--
Roger