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In article , "Helen Findlay"
says...
There has been no work done to the house in at least five years.

The floorboards upstairs in some areas seem looser and move noticeably when
walking over them

That could be shrinkage caused by central heating reducing humidity.
If that's the case they can be probably be secured by screwing down,
being careful to avoid plumbing and wiring that might be underneath.

and downstairs there is a slight incline where they seem
lower than the rest of the room.

That could be wet rot or similar damage to the joists, usually where
they are supported by a masonry wall. Best to lift the boards to have
a look.