Thanks very much for the additional info, CarefreeBA and US HK UK Flyer. I've also read that the exchange rate one gets when adding funds to the tourist Octo app can be really poor (and presumably this applies on refunds too) but then, considering again that I'm only there for a few days and probably would not put much money on it, this would not amount to anything significant, so perhaps the convenience of the app is well worth a few measly dollars lost to FX.