If you are flying into Costa Rica from any South American country, you should check to see if there is yellow fever CURRENTLY in that country. You would need a vaccination if you are flying into Costa Rica from a country that has a yellow fever warning IN COSTA RICA. Note that you will need to get your yellow fever vaccination 10 days ahead of your flight.
When I flew into Costa Rica from Colombia, there was no yellow fever there and no yellow fever warning and I did not have a problem. I did visit Argentina where there is a yellow fever warning in one small part of the country - but - I did not visit that location. And upon arrival, no one asked me about where I went in Argentina or even mentioned yellow fever vaccination.
However, Sally S, the OP, is flying into Costa Rica from Northampton, which, the last time I looked, is not in South America. So really, all the warnings and angst about flying in from Colombia don't belong in this thread.