It's probably the National Museum of Wales in the city centre, I doubt if they would go to St Fagan's in this weather. I don't think the underground car park is terribly close to the stadium, although they've changed that part of the city recently and I very rarely go in nowadays, I refuse to take the car and struggle to find a space in a car park, and now my mobility is restricted it's difficult to get to and from the bus stops, let alone stand waiting for a bus. They will stay overnight before going home?

Paula x