The downstairs toilet used to flush OK but now the bowl fills with water
and then takes about five seconds to drain away, the water level
dropping about 3 cm below normal and slowly rising again to about 1 cm
below normal.

It seems to me that there must be a partial blockage somewhere, and I'd
prefer to sort it out before it becomes a total blockage. Any
suggestions?

From the back of the pan the pipe turns right, goes through a side wall,
then turns right again and goes into the side of a vertical soil stack
(total distance so far about 1 metre). The soil stack is only about a
metre long, being capped just above the junction. This unusual (and
possibly illegal) arrangement hasn't given any trouble before in the 15
years we've lived here.

--
Mike Barnes