Is public transport straight forward for getting from BHD to Bangor?
Yes. Sydenham railway station is across the A2 from the old passenger terminal. When you arrive ask at the information desk for a shuttle bus to take you down to the station (it's inaccessible on foot because of the freight and private jet terminal). A bus will take you down there, and it's across the footbridge. Platform nearest you for trains every 20 / 30 minutes to Bangor. Times / fares at
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On the return, there's a small shelter inside the airport gates by the footbridge. The courtesy phone inside is broken, so use your mobile to call the number on the shelter to call a bus. Think that number is 028 9093 5105.