There's no need to book a hotel to be honest. Maybe if you are arriving there late at night, book one night somewhere. If you arrive first thing in the morning, get the bus/taxi into Bangkok then sort out your entire trip with a TAT office. They can arrange everything for you, cheaper than doing it through a UK agent. They probably take a bit of a cut but mig be worth it if its your first time there. Next time we go we'll book flights then do it ourselves there. Easy to do as the guys above have said.