I got the A1 bus without too much trouble and to amend some internet advice elsewhere, the bus stop is now right by the main Arrivals door, slightly to the right. I did take out 1,000 dinars from the ATM - my one didn't charge a service fee, and it came out as 2 x 500, and in any case the A1 driver was happy to accept higher value notes. My first van was full, just as I arrived, the next van arrived fairly quickly, though it's a 20 minute service interval, and wasn't full. Did have toddler nearby so I learned more of the Serbian version to Baby Shark than I would have liked, but it did the job. I took it since my hotel was right in front of the Belgrade terminal stop. I used a downloaded Google map to track the journey.