Thanks for the advice.
I have located this on the Swiss website:
Terms
There is always a charge for transporting sports equipment regardless of the baggage allowance on SWISS flights. Sports equipment of the category “Small” is excepted provided it does not exceed the baggage allowance.
Although my folding bicycle fits the dimensions of the "small" category, the baggage table goes on to read that bicycles are a "Sport Equipment with fixed categorisation" in the medium category, meaning that a charge applies.
However, elsewhere on the site it reads
Your checked sports equipment is free of charge if within your free baggage allowance.
Does anyone have some recent experience of trying to fly with a bike within the luggage limits?